You just need to specify the fully-qualified classname, i.e. packagename.ClassName
. So, if I understand your project structure correctly, your controller class is scaterChartController
1 and it is in a package called testPac.view
1,2. So your fx:controller
attribute should have the value fx:controller = "testPac.view.scaterChartController"
.
In SceneBuilder you can set this in the "Controller" pane which is in the bottom left of the screen (expand it if necessary):

Footnotes:
- You should follow proper naming conventions, so all class names should begin with an upper case letter.
scaterChartController
is not a proper class name according to the standard convention. Similarly, package names should be all lower case, so "a package called testPac
" also violates the convention.
The package name is evident from the first line of code. I am assuming you have a package name of view
, so the first line of code in the controller class will be
package testPac.view ;
Modify the fx:controller
attribute accordingly if the package is different to that.