I also run an iMac. You don't say what OS you are using. If you are using OS X, then go to Home>>Applications>>Limewire>>Shared and you should find your files.

If you are running OS 9.x, then click your hard drive icon, go to Applications>>Limewire>>Shared. You should find everything there, unless you specified another folder when you initially set up Limewire.

Try using Sherlock and do a generic search for all your .mp3 files--it will give you a file tree that will show you where they are all stored. I've upgraded every time Limewire puts out an update, and I've never lost a file. So I am convinced they are on your hard drive somewhere.