I copied this from the FAQ page on the LimeWire website. You should give this a try. I hope this helps.

Q: I am trying to install LimeWire on the Mac, and I keep getting an error message saying that the installer is corrupt. What can I do??

A: Increase the preferred size of the installer in the "Get Info" window for the installer file, and then run the installer again. If the installer gives you another error, reboot your computer and run the installer one last time.