It is important to note that we need to distinguish development-time and run-time behavior.
For run-time, the jars should be in the IS class path already, hence no need to add them to the package under normal circumstances.
For development-time, though, Designer needs to be made aware of their location. More specifically, you need to add them to the build path of you Designer/Eclipse project. This happens just like with any other Java project.
Hope that helps.
Best regards,
Christoph