I HAVE THE SAME PROBLEM. Sorry for the caps but now that I got your attention you might as well read the whole post.
I entered my Halo (mac version) CD-key on the nameplate thingy and it said that it was entered correctly and something blah blah blah so I refreshed the page to see it really worked but NO, IT'S GONE NOW!
Now when I try to re-enter the CD-key, it tells me it is already used but I CAN'T SEE IT ON MY PROFILE! Seriously, I have already PMed the original poster about this, sent e-mail to the support but it's been 4 days and no answer. Can someone PLEASE help me?