From what I can see you may need to port forward your router device. Viewing Motorola SBG900 - recommend? - dslreports.com and also SURFboard SBG900 - Motorola it appears there is no UPnP support however I'm not sure about present models.

If port forwarding. Always very first step is setting up a static ip address Setting up a Static ip address on OSX.

Then Port Forwarding Limewire on the Motorola SBG 900 however the LW 5 equivalent settings are here: Manual port forward instructions & sample image.

I presume you made an allowance for LW in your OSX firewall? OSX Firewall settings for Leopard