Run the checkout and ant build steps from the regular JythonDeveloperGuide
In your Eclipse preferences, go to Java > Build Path > Classpath Variables and add a new variable named ANT_HOME that points to your ant install. That directory should have a lib/ant.jar inside of it.
Go to File > Import > General > "Existing Projects into Workspace" in Eclipse
- Select your Jython checkout directory as the root
This could be made to work entirely in Eclipse with a few more additions to the external builders. For now, it requires running ant in the first step to make the build/gensrc and build/jarjar directories and then every time after a clean.