I rephrase my reply.
You may wish to compile a plug-in for Reader but you must use only those features available in Reader. This means that some of the samples, including this one, cannot be used since they are not for Reader. Therefore to compile this plug-in it is wrong to follow the instructions for creating a Reader plug-in.
There is no separate SDK for Reader. Are you trying to develop plug-ins without owning Acrobat? You will need Acrobat to do the development.