Xbox wireless internet troubles....help?

okay, i have a scenario, i don't have a wireless router only a modem. but i can use wireless internet because of the internet sharing on my mac. i want to connect my xbox to the wireless internet. on my xbox, i have a wireless internet adapter. but for some reason, when my internet from my mac is sharing, i cant connect to xbox live.? understand.? how can i get wireless internet to my xbox from my mac internet sharing. i already configured my internet sharing on my mac so that's out of the question. when i go to connect to xbox live it says it can't obtain an IP address. ? i need help...!