If you are looking for pydev on Aptana 3 beta, refer here: https://yuji.wordpress.com/2010/06/11/aptana-3-python-support-pydev-on-aptana-3-beta/
I’ve done this a few times, but each time I forget the process. I always end up at the PyDev site and then find my way in Aptana.
You just need to go to Help > Install New Software
In the tooltip that pops up, hit hte checkbox next to Aptana PyDev, and follow the instructions as you continue through the guided installation.
By the way, I had an issue where the default editor for .py was set to something other than the “python” editor. Make sure it has been set.