Okay, after a few days of research and trying different options, I read from another BMW forum that the dealer carries specialized mounting brackets for the rear trunk CD changer option. Since the Alpine MP3 changer is the same size as the Alpine CD Changer that the dealer sells, it fit exactly the same.
After talking with the actual parts dept at the dealer, I found out that the mounting brackets were only $25 instead of $350 which the salesman told me. Needless to say that I bought the brackets which included all of the screws and bolts too.
The directions that come with the mounting brackets are not clear at all, but after playing around with the 2 brackets and figuring out how they fit in the trunk space, the rest of the installation was quick from there.
Anyhow, don't want to bore everyone with the actual details, but if anyone wants to hook up a 6-disc MP3 changer, there are additional pieces of hardware and cables needed to interface with the factory radio. I can post that info later if anyone is having the same confusion that I had a few days earlier...