Glad to see you got it connected at least, but are you using the method I suggested, or are you using ICS? In any case, here is another cut and paste from my page.
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"Xbox live MTU Fail?"
"How do I change my MTU setting?"
"My xbox fails the MTU test"
To have a better idea of how to fix the MTU, it's good to understand what it is and what it stands for. MTU stands for Maximum Transmission Unit. You likely understand that the internet consists of little 'packets' of information flowing everywhere, instantly. The MTU is a setting that says how 'big' one of those packets can be. The larger the number, the bigger the packet, and therefore the more bandwidth can be used in one packet. For Xbox live this setting has to be a minimum of "1364."
This setting is usually set in your routers' configuration screen. It will be in some sort of advanced configuration screen. This is where I say the typical 'see if your router lets you change this setting'.
Now, just because your router says it can be changed doesn't mean you're out of the clear. If you are in a dorm, or another sort of shared internet setup you are likely out of luck. The MTU is set for a smaller size for the entire connection and cannot be changed. If it was set to a large MTU, then there would be serious connection issues as some people hog the entire amount of bandwidth to download large files.
Also, some routers need a firmware upgrade to support the MTU setting and be able to connect to Xbox live. Check your routers homepage, look up the downloads for the router you have and see what the latest firmware driver is. You should be able to simply download and install it from there. Few ISP's have routers that simply will not support Xbox live and need to replaced. I've run into this problem especially with AOL Broadband customers who have been customers forever and have their early routers. Below is a link to a page at Xbox.com listing 'certified' compatible routers, but more importantly, it lists 'incompatible' routers.
You can find that list below.
http://www.xbox.com/en-US/support/connecttolive/xbox360/homenetworking/equipment.htm