After you download and install it, open it up and you'll get a randomly assigned username.
After you do that, go to the link above ^. Change your gamertag to whatever you want your MW10E username to be. Claim the gamertag. At this point, you should be logged into xbox, if not, log in. Once you're logged in, log out and log back in. Make sure you have MW10E closed. Then re-open it up and it should change to whatever you changed your gamertag to.
~ made this post because lots of people were having trouble with this, credit to a mod for the link to change your gamertag.
The Meaning of Life, the Universe, and Everything.
I have an xbox account but i can't change my name
i can sometimes open it fast enough to be able to type something but it disappears before im able to press "done" and if i am able to press done it just greys the box out again with the old name..... seriously? facepalm.