I have already installed the eclipse IDE for Java developers (Helios Service Release 2)
How do I now install the Plug-in Developers Environment (PDE) on top of that? I thought that it should be possible to install it from an update site, like any other eclipse plug-in. But I can’t find the correct update site anywhere.
Try that in Eclipse:
Help → Install New Software… → Choose The Eclipse Project Updates in field Work With → Filter for RCP Plug-In
Then press Next and follow the instructions!