Plug-in can't instantiate class

There are version, chapter and step numbers for a reason. Since you omit all the useful information in your question, we can only guess what you are actually doing in what context. Therefore here the general answer:
That error is not related to the extension development, that is related to the Eclipse you are using. Try Luna or try a different packaging of Eclipse. As Donald mentions there are Eclipse forums where you might want to ask, Stackoverflow is also very useful to search and ask.