LW may have automatically done this: each machine needs to run on a separate port. You can check which port each is using by looking through the Preferencess->Bug Reports->View Example

Then just search for a file you know is shared by the other computer. If it should show up quickly as a blue entry, you're set. If not, firewalls and port-forwarding may need attention.

This can get technical, so let me know if I'm being too simplistic or obscure.

To ensure each Limewire to use its own port, set the Mac [OSX?] to, say, port 6346 (set in Preferences->Advanced->Port, and Firewall). See that the PC [?] uses something else (like 6348, for example).