I wouldn't bother. It claims to defragment virtual machine files, which is a non issue on HFS formatted partitions. Macs don't need to be defragmented. Further, we have 'disk cleanup' features built into the product.
If you really want to save space, defragment the guest and then run guest-cleanup, tho I don't know how mcuh of a benefit that will yield over guest-cleanup itself.
That being said, I've not heard of others experiences with this tool and I'm not sure how old it is, so it may or may not even be safe to use.
Could always try it on something you're not worried about corrupting and let us know your results 
