Did you download the right version of Limewire? Be sure to get the Mac OS X version. Don't just download the Windows version and expect it to work.
You should also check out various torrent programs. One good one for OS X is Transmission.
Now before anybody goes on a tirade about piracy, it IS possible to use P2P programs for legal purposes. Since the OP has not specifically stated that he will be using these programs to do something illegal, we cannot make such an assumption. Therefore, there is no reason to get uptight about "violating rules" until Dlee45 specifically states he will be breaking the law.
There is plenty of legal stuff on Peer-to-Peer networks. It's a good way to get discovered. Of course it's abused and made into Pirate-to-Pirate networks... but we shouldn't make assumptions about a person beccause they want the program.
p2p is pretty much newsgroups for the new generation. sure there may be some legal stuff there, but come on now...
plus if he's just looking for software, a place like versiontracker should be fine, plus it will give him lots of info, will have the most up to date versions, and can keep you updated on newer versions as they are available.