If youre having trouble follow the steps. Someone posted this on xbox360fanboy.com. It helped me.
1) If you are logged into xbox.com, log out!
2) Once logged out click this link:
http://xboxrewards.msgamestudios.com...t.aspx?lc=1033
3) Click the Green "Sign In" button.
4) This was the tricky part that got me screwed up many times. Do not sign in here, instead, click the Green "Register Now" button on the bottom left of the page.
5) Now click you can click "Sign In".
6) Fill in your proper information and click "Submit Registration".
7) Now it should say "Registration Successful. Thank you for participating!" Some people thought it was time to get there game on from this point but nope, you need to now join the challenge. The link below is the quickest to join the challenge. Don't click "Return to My Xbox Rewards" from this page.
Direct link to challenge:
http://xboxrewards.msgamestudios.com...tail.aspx?id=4
Click "Join Challenge" a small box will pop up on the page and accept and you are now ready to get your game on.
8) When you have your 1,500 points in the bag, click the direct link to the challenge link above again and click “I’m Finished” and it should say “Congratulations”