Here my comment as answer again:
OK, found the solution: it's a PDE problem, see accepted answer in Eclipse OSGi bundle reqiure another bundle's fragment
After changing the manifest of my guice-3.0.jar in the target platform like described in the link above, I needed to do the following:
- Reload the target platform (Windows -> Preferences -> Plug-in development -> Target platform) and restart eclipse.
- Remove the bundle com.google.inject from the required bundles of my plug-in.
- Add all needed com.google.inject packages to "imported packages" of my plug-in. Now in the list of packages I can see com.google.inject.assistedinject.Assisted and import it!