I installed eclipse and then wanted to install pydev plugin to execute python programs. But it gives an error "pydev not found at https://pydev.org/updates" .
How can I overcome this? And is there any other any alternative?
You should be able to install it through Eclipse. The PyDev page has the instructions on how to do this.
If you are not using Aptana, scroll down to Installing with the update site and start there. You'll start by going through Help > Install New Software...
I just did it myself on 12.04 and so far everything seems to be working fine.