If those myNamespace_myModulname are really what you have in your actual code, please first have a look at @fantasticrice answer
Then, if we consider your code with the right naming convention (see note in the end of this post), to start up with, this is not a valid block class :
class Mynamespace_Mymodulename_Block extends Mage_Core_Block_Template{ /* ... */ }
If, like you say it later on, your file is located in
app/code/local/Mynamespace/Mymodulename/Block/Index.php
then the valid block class is
class Mynamespace_Mymodulename_Block_Index extends Mage_Core_Block_Template{ /* ... */ }
Because class name in Magento should always follow the exact same architecture as your path to file (except for controllers, but we are not talking about controllers at all with the code you gave us here) =>
class Mynamespace_Mymodulename_Block_Index === Mynamespace/Mymodulename/Block/Index.php
(see how I just replaced underscore with slashes and postfixed this with a .php extension ?).
Then what you actually want is your your view mynamespace/mymoduleoutput.phtml to use your own module block.
For this, you have to specify the right type for your block.
The type is defined by a combinaison of your handle defined in config.xml and from the path to your block file.
1. The handle
When you define a config.xml for a module, you have to be aware that some part are "fixed" and some parts are "handles".
Meaning that some parts are expected by Magento to be in a limited number of possibilities e.g. frontend or adminhtml or global, blocks, models, helpers, ... there is more here and some are just name to alias your module and to call it or handle as we call that under Magento.
Here you say in your config.xml that for the <global> config (means for both frontend and backend (= admin)) -- fixed -- you want to add to the list of existing <blocks> -- fixed -- of Magento the blocks of your module that would be referenced by the -- handle -- <mynamespace_mymodulename> which will then map to files in which the classes will all begin with Mynamespace_Mymodulename_Block.
That is the first part of the type you want for your own module block.
mynamespace_mymodulename would be equivalent for Magento to Mynamespace_Mymodulename_Block
2. The right block
Then you just have to indicate the right path from the root folder of your Blocks to Magento, which will be the exact same as your folders/files architecture, once again : so in your case, just index.
So as you may now understand, Magento will lookup to a class Mynamespace_Mymodulename_Block_Index (handle + '_' + specified block) in the file Mynamespace/Mymodulename/Block/Index.php as seen earlier.
But if your file was under app/code/local/Mynamespace/Mymodulename/Block/Path/To/File.php you would have path_to_file => class Mynamespace_Mymodulename_Block_Path_To_File, file Mynamespace/Mymodulename/Block/Path/To/File.php
Now that we have the second part of our type we just have to assemble them with a slash : mynamespace_mymodulename/index
So you have to change your call in your cms to :
{{block type="mynamespace_mymodulename/index" template="mynamespace/mymoduleoutput.phtml"}}
And hopefully, with this, it would work.
NOTE 1/3 : As pointed out by @fantasticrice, I would also suggest you to follow the naming convention of class:
Zend Framework standardizes on a class naming convention whereby the
names of the classes directly map to the directories in which they are
stored. (...)
Class names may only contain alphanumeric characters. Numbers are
permitted in class names but are discouraged in most cases.
Underscores are only permitted in place of the path separator; the
filename "Zend/Db/Table.php" must map to the class name
"Zend_Db_Table".
If a class name is comprised of more than one word, the first letter
of each new word must be capitalized. Successive capitalized letters
are not allowed, e.g. a class "Zend_PDF" is not allowed while
"Zend_Pdf" is acceptable.
These conventions define a pseudo-namespace mechanism for Zend
Framework. Zend Framework will adopt the PHP namespace feature when it
becomes available and is feasible for our developers to use in their
applications.
See the class names in the standard and extras libraries for examples
of this classname convention.
Source : http://framework.zend.com/manual/1.12/en/coding-standard.naming-conventions.html
NOTE 2/3 : that per convention again, only class are going to have a capitalised file name / folder structure. So, your template folders and files should not have capital in it at all.
NOTE 3/3 : In Magento, again, per convention we do not use capitals in handles.
TL;DR
app/code/local/Mynamespace/Mymodulename/etc/config.xml
<?xml version="1.0"?>
<config>
<modules>
<Mynamespace_Mymodulename>
<version>0.1.0</version>
</Mynamespace_Mymodulename>
</modules>
<global>
<blocks>
<mynamespace_mymodulename>
<class>Mynamespace_Mymodulename_Block</class>
</mynamespace_mymodulename>
</blocks>
</global>
</config>
app/code/local/Mynamespace/Mymodulename/Block/Index.php
<?php
class Mynamespace_Mymodulename_Block_Index extends Mage_Core_Block_Template
{
public function getCollection()
{
// some code
return $collection;
}
}
CMS content
{{block type="mynamespace_mymodulename/index" template="mynamespace/mymoduleoutput.phtml"}}
app/etc/modules/Mynamespace_Mymodulename.xml
<?xml version="1.0"?>
<config>
<modules>
<Mynamespace_Mymodulename>
<active>true</active>
<codePool>local</codePool>
<depends/>
</Mynamespace_Mymodulename>
</modules>
</config>
app/design/frontend/base/default/template/mynamespace/mymoduleoutput.phtml
<?php /** @var $this Mynamespace_Mymodulename_Block_Index */ ?>
<?php foreach($this->getCollection() as $item): ?>
<?php //do something ?>
<?php endforeach; ?>
