Have you ever had success with using LW 5?
If not you will need to downgrade; Downgrading to LimeWire 4 from 5 & resuming incomplete downloads

Reboot into safe mode, holding down the shift key until the apply icon shows. Then log into your account. Open Disk Utility & repair permissions. Reboot normally. See if that helps.

Make sure Java 1.5 is somewhere near top of the list: Setting up Java for Leopard (see 2nd image)