Internet Sharing through Ethernet not working

My friend set up his Macbook to share his Internet from Airport through Ethernet, and he connects that to his Xbox 360. I am trying to set up the same thing, but nothing seems to work. I use this guide ( Share Your Mac’s Wireless Airport Network Connection With Your XBox 360 ), which is a tad dated, but the general idea is still the same, and it worked for my friend. When I connect it to my Xbox, the Xbox says it can connect to the network, but not to the internet. Any clues?