Yes, I believe it is possible. Is it a lot more work than necessary when a <40 router will do the job, and will allow you to use your macbook wirelessly also.
If you do want to do it the hard way, you need to plug the ethernet cable into your macbook. make sure it can access the internet. then you need to turn on internet connection sharing in the sharing section of system preferences. then allow it to share your connection from ethernet to airport. Click "airport options" and configure a password. Then turn on your xbox. Connecting to the newly created wireless network should be the same as connecting to a normal wireless network.
Note: I haven't done this specifically with an xbox 360, but I have created a wireless network on the go sharing my bluetooth modem with another laptop while traveling in the car. not too bad with EVDO rev a.