I cant get my xbox 360 connected to live because of the MTu i dont know how to set it to 1364?

I have had my xbox 360 for 3 days or so now and iv was playing COD Black Ops this morning and then all of a sudden i disconnected so i tried to get back on live and it didnt want to work i tried everything i knew and wat xbox.com told me but the only option i didnt try is MTU wich needs to be set to 1364 i cant find how to change it because its my step dads adsl and iv never had a problem with it so i dont ever realy go near it could anyone help me to change the settings to 1364 so i can carryon playing COD on xbox live :D