Okay, I didn't get it at first, but now I think I know what your problem is, thanks to the post above.
You can hold down a key while starting iTunes to make it let you select the library location (this is different from setting it in preferences -- when you set it in preferences, it starts storing new files there; when you do it this way, it looks for a pre-existing library there).
In OS X, the key is Option. I think the key in Windows is Shift. So try loading iTunes with the shift key held down, and then navigating in the dialog to the Macdrive copy of the iTunes library.