this link which i gave you is a sure shot way to make it work.
I hope your network settings say that this is a private network so that you are alowed to share your media.
Also do you see your xbox as an extender in your network places?

i am just not sure why this is not working for you? i tried the same by disabling my WLAN connection and then dialling through a dialup connection so i just do not know what is wrong in your case.

sorry can't offer any more assistance here