Head over there to sign in and register your gamertag.
Dont know what this is all about? Details here
Edit : As you may have noticed, the site is experiencing some issues right now. The marketing team is working on fixing them. In the mean time, if you are able to registeryou then have to join the challenge. So really its a two step processassuming you can actually get into the site. Once I get an update, Ill report back.
Edit: This statement just in from the Marketing team:
Thank you for the incredible response to Xbox Rewards. The gaming community’s interest has temporarily overloaded our servers and caused delays for those trying to register.
To help ensure everyone who wants to participate has the opportunity, the Old Spice Experience Challenge will be extended by three days. Now you have until April 15th 2007 to increase your Gamerscore by 1,500.
As a bonus, we are adding more rewards for this challenge, which means more gamers who successfully complete the challenge will get rewarded.
Our development team is hard at work to fix the problem. We expect the web site will be ready in 48 hours, please be patient and check back periodically.