You can register at the CrossFit Games website.
Click the register button and you will be taken to Step 1 of the registration process.
Fill in the proper person information in the "About Me" section, select an affiliate you would like to make a contribution to, select the amount you would like to donate or choose to participate for free, and agree to the terms and conditions.
After you agree to the waivers and pay the contribution you are all signed up!
Good luck and enjoy!