The Error Message
I am note sure why you are getting an error message about content.xml, the real file that Eclipse should be reading is content.jar which is there.
Try one of the solutions below and please post more details if you can.
Online Install
The URL for installation of the feature is http://pydev.org/updates. You can use that URL to Install New Software... from the Help menu if the Eclipse Marketplace is not doing what you want.
Offline Install
If you meet the requirements (below) and continue to have problems with the update site, you can download to install offline. Get it from SourceForge (or follow the link from PyDev's website to SourceForge)
LiClipse
Alternatively, simply get LiClipse instead. It is Eclipse and PyDev and a bunch of other useful stuff already packaged together. It is provided by the author of PyDev, so should give you the best experience.
Minimum Eclipse and JVM Requirement
Double-check you meet the minimum requirements: PyDev requires Java 1.7 or greater and Eclipse 3.8/4.3 or greater: