How do I add my XBL gamertag to this site

I have Dragon Age for the PC. When I created an account on this site I figured "why give them my XBL gamertag, I can always add it later should I feel the need". Well, that need has risen in the form of I need my ME2 armor and gun. the problem, I can't seem to figure out how to add my gamertag to my profile on this site. Somebody help me!