As far as I know, the problem is that the IDE does not compile the .CPP files. It willhappily include a .H files that you point it at.
I'm not 100% sure, but the directories where it will look for .CPP files are basically predefined (Arduino installation libraries, sketchbook libraries directory and sketch src directory).
There was a similar question not too long ago (if not mistaken) and the solution (again, if not mistaken) was to put a symlink in one of those directories.
An alternative is not to use .CPP files and place the code in .H files; not 100% satisfying in my opinion.