You don't need to!!
OS X does it for you.
If you leave your PowerBook on overnight every once in a while, it does little janitorial things while it's idle... I'm not sure on what they exactly are, but I'm sure it includes defraggin'.
Enjoy your Mac! PowerBooks (and the whole Mac line) are AWESOME!
**eDIT**
PlaceOfDis: Get out of my reply window! haha