It may come formatted as FAT32, which enables it to work with any computer straight out of the box.
When you first plug it in, launch disk utility and initialize the Drive as Mac OS Extended (journaled).
now the question is; is it compatible with your Mac?
You may want to share your Mac information with us as some computers, notably some G3 and G4 versions, do not recognize drives larger than 123GB
Is the new Lacie Rikiki 500 GB USB 2.0 compatible with a mac OS X 10.4.11?