You don't, HFS+ is a modern operating system and doesn't need defragmentation.
Interestingly, PC World (I'm a about to Switch person) did a test of before and after defrag test, and found that these days, defragging doesn't really do anything performance-wise. I can see how the Mac position (on the link provided earlier) is correct in this regard.
Quite true.
Defragging was great when disk speeds were low, so seek time made a difference. Now-a-days, with 7200RPM drives (or faster), that's become a rather moot point.
You don't, HFS+ is a modern operating system and doesn't need defragmentation.