(Starting to summarize how to solve those frustrating Eclipse problems, that come up over and over again ...)
... then, Click the little validator icon in the product file (right left to the question mark icon). Add the mission plugins/features in the corresponding tabs (plugins/features) in the product file editor (the tabs are at the bottom of the editor).
So, the things you have to know how to do, sometimes over and over again:
Also, when importing jar files into the build path of a plug-in, you have to include the jar file itself, it does not work with just the folder that contains it.