OS X :: Trying To Defragment The Hard Drive Using Snow Leopard?

I am trying to defragment the hard drive using snow leopard but I get the following message:You cannot use this algorithm on the selected volume. Off-line algorithms need to unmount the disk so you need to boot from a different disk, partition, computer or CD/DVD. Alternatively, use the Quick (on-line) algorithm.How would I do this without doing a quick algorithm? Basically how do I unmount the disk or boot from a different disk or DVD and run this program, iDefrag.