Here's the way to do this, for those getting caught on the birthdate thing:

Click Sign in.
Select a language.
It will tell you to update your Windows Live Profile with a birthdate. Click on Update my Profile.
Double-check your profile, make sure it has everything required -- not only for the form, but also enough info so that you can receive your package!
Save the form.
It will take you to a form almost exactly like the previous one. Double-check it for the same things, click Save.
Click on Return to the Forerunner Site.
Enter code.
Be happy.
DON'T ENTER ANY MORE SERVER CODES -- Let others have a go!
Play ARG.
Get package.
Pee pants.
Continue to play ARG until Halo 3 blows your mind into kibbles 'n' bits.