One more time - you don't need to - OS X handles this quite well on it's own.
Just checked my system last night, been running it for almost 2 years now without defragging or a fresh install. It was 0.6% fragmented. And I install, remove and modify tons of files on an ongoing basis.
Unless you're hitting the point of less than 25% free space, a defrag is typically not going to help with OS X. If you are hitting that percentage, I suggest freeing up some space. If you're insistent that you want to defrag, you can grab the app iDefrag.