Is your Motorolla set up to use UPnP? Telnet into your motorolla using http://192.168.100.1 to check.

Do you also see the brick wall on your windows machine?

This is my suggestion, port forward for each computer. You'll need to do it "from" each computer because port forwading is based on their ip address on the network. And each computer will "need" a different port number to use for LW. Port Forwarding for the Motorola SBG 900

Static ip instructions for OSX are (a) http://www.blizzard.com/support/wow/?id=aww01180p or
use (b) http://www.sonic.net/support/ss/mac/osx_staticip/ (ignore the Domain boxes)

So what ports to use? For windows, use port 6346. For mac you could use port 64049.

You should restart both LW's after the port forwarding process. Make sure you save your settings in the motorolla. Some devices you need to reboot them to properly save the settings & get them into action. Not sure about yours.