There have been some popular posts in this blog describing the use of Eclipse, PyDev and monkeyrunner. However, I have received a lot of questions regarding the definition of the interpreter and perhaps it requires a little deep explanation.
Even though monkeyrunner is a Jython interpreter, to successfully use it from PyDev you have to identify it as plain Python.
This screenshot shows the definition of the interpreter which coincidentally, we called monkeyrunner.
Following these steps you would have no problems using monkeyrunner from PyDev.
Hope this helps.