I don't know exactly, so just check ll your setings and FireWall.
Here one man's setting and a ling, where you can find detail information step by step how to connect your xbox to Mac.
But in fact I think you just miss smth with your Firewall.
I'll try to find else solution if it still doesn't work correctly.
You must first go into System Preferences > Sharing and enable internet sharing.
Then, if you want to, disable your firewall (System Preferences > Security > Firewall). If my firewall was not disable XBox Live gave me a NAT error, but still let me connect.
Network Settings:
Manual IP/Manual DNS
IP Address: 192.168.2.2*
Subnet Mask: 255.255.255.0
Gateway: 192.168.2.1*
Primary DNS: 192.168.2.1**
Secondary DNS: 192.168.2.1**
*Run “ifconfig en0″ in the Terminal, use the IP address returned after “inet”. I’ve been told that it’s always 192.168.2.1, but I’m not sure. For “IP Address”, add 1 to this number, for “Gateway”, use the number.
** This is the same number returned using “ifconfig en0″. Others have said to use the DNS of the ISP or the router, this has never worked for me. Instead, using the IP of the Mac did.
and a link
Share Your Mac’s Wireless Airport Network Connection With Your XBox 360
Write if your need smth else.