I finally found the solution of this problem: For some reason the cpp file implementing the PortMixerNative class was not included in the project. After adding the PortMixerNative.cpp to the project again I finally could see that there were syntax errors in this file. After fixing the errors and recompiling the .Native project I could successfully reference the class.
The strange thing is that the .Native project successfully compiled even with methods without implementations. If anybody encounters this problem too, check if all methods of the class you try to use have an implementation.