Make sure you are using ContolP5 2.0.4.

See: Examples > Contributed Libraries > ControlP5 > extra > ControlP5frame

I can confirm that processing 2.0, controlP5 2.0.4 works fine in eclipse. Have you made sure you are linking to the required jar files in your build path and not any older versions? 

Thanks guys for answering that fast. I found the problem: If I use the .jar file I still get the error message and ControlP5 says it's version is 0.1.1  in the first message. If I import the src files, everything works fine.