Take a look at my thread here:
some more ideas/issues
About halfway through there will be a link to a zip file of the sample project. I couldn't get anything to work when trying to start from a new class library either. I downloaded that sample and opened it and when compiled it would work. Don't know what the difference is between that and a new class library with the correct references but it worked. I then just copied the whole project to a new folder, opened the new copied project sample, and renamed all instances of OMPluginSample with what I wanted. It works for me at least .
I also put my plugins in the skins folder instead of the plugins folder and they appear in the list for the main menu.