I'm stumped. Have you checked to see if any OSX updates? Have you repaired permissions after a safe boot? SAFE Boot & Repair Permissions
So LW 5 installs but the actual program as a cross through it? I'd have thought your imac would be ok. Perhaps a problem with the original OSX installation which I believe is what my problem was. I think mine needs a fresh install wiping drive at same time. I did archive installs last year to no avail for LW 5. But my LW 5 works fine in a different OSX installation on a different drive.
Only answer I can think of is downgrading to LW 4.18. To do that you will need to remove install limewire.pkg file from the Receipts folder. Receipts folder sample image

What happens with LW 4.18 & 5 on my main OSX: http://www.gnutellaforums.com/attach...-dead-nail.gif; not even a multi-bounce, simply dead.