HFS+ (at least in 10.3, maybe in earlier versions of Mac OS) is kind of self-defragmenting. Whenever a file is opened a set of conditions are checked which are something like is the file fragmented and less than 20Mb and not a heap file. If these conditions are true then the file is moved to a new location on the disk that will result in it being defragmented. So you don't need to defrag.