[TennsDog]

Ok, I think this will help you (and hopefully get you where you want):
-Load iTunes with the external drive on and connected
(-make sure the library is exactly as you want it)
-go into iTunes preferences and make sure the Library folder is set where you want it, save/close preferences
-click File > Compile Library

What this does is goes through and locates all of the media in your iTunes library, copies it over from the original location (in your case, your external drive) to the designated Library folder (your internal MBP drive), and then, and most importantly, redirects the file paths in the iTunes files to tell iTunes where to read the files from.

This will/should do what you want: transfer your library from external to internal and keep it intact. All the music will still be on your external (your external will not know anything changed), but iTunes will no longer be using that at all for reading media. I don't, however, know of any way to have the same library linked across computers like it sounds like you want. If you do this on your MBP and change nothing on your PC, the PC iTunes should still be reading from the external, so it will be the same for practical purposes. It will, however, be a copy and not actually refer to the same stack of information. If you change something on your Mac, it will not affect the PC and vice versa. Hope this helps.