Is that plugin an 'inplace' plugin? Because it doesn't look like it has the correct icon of an inplace plugin (should look like a link).
Anyhow, if it is an inplace plugin then I don't think you will get the source code linked directly under there. This is because the inplace plugin is represented differently: as a eclipse project dependency. The node you see is just a 'fake' to show you that you have a dependency there.
You should also have this project in your workspace... so just navigate to it and put breakpoints in the code there. That should work... but let us know if it doesn't. Could be a bug we need to fix :-)
Kris De Volder -- SpringSource