I hated playing my 360 with a wireless connection. I even moved my router into my room, and left my desktop connectionless (I have a wireless pci card I'm gonna add to it), just so I could play on Live on my connections full potential.
About your problem, I also only know how to do it on a PC. It sounds like you have all the materials you need, but just need to figure out the settings in your connection manager on your Mac
Good luck.
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