You should not need to manually install Java 1.6, mac update should do it automatically. To see if your computer is compatible with LW 5, go to Java preferences & see if in fact Java 1.6 is installed: Setting up Java for Leopard (see 2nd image)

If no java 6 listed on your java preferences, your computer is not compatible with LW 5. LW 5 needs both OSX 10.5 or later & Java 6. And java 6 being 64-bit can only be installed on the newer intel machines.

I hope you still have your LW 4 Pro installer just in case. If you need to downgrade again, you will first need to remove the instal limewire.pkg file from Receipts folder; Receipts folder sample image