How do I "allow" my gamer profile xbox live rights?

I only have 1 profile on my xbox. Whenever I try to play halo3 on xbox live it says "matchmaking requires an xboxlive membership with permission to play over LIVE"whenever I go to xbox live settings it says "there where no xbox live child profiles found"How do I "allow" my gamer profile xbox live rights?you might have to chage the setting and change it to say that you can play M rated gamesHow do I "allow" my gamer profile xbox live rights?if you just complete a few quick tasks on this site they'll send you an xbox 360, completely free of charge!!