Mac OS X & Mountain Lion Community
MacBook Series Forums
Using Font Book: http://support.apple.com/kb/HT2509
I think the makers of word decide to include "the correct" set of fonts, and present that as the original set available.
I believe you can then extend that set by deliberately adding your own plebeian fonts should you choose to do something so décalssé, but they did nothing to make it easy. There may be commands on a menu somewhere that allow you to add fonts, one at a time.