Ground(ctrl)
c/o Adam Lambert
120 K Street
Suite 300
Sacramento, CA 95814

It also states the following which is worth noting:

Please keep in mind, it is highly unlikely that Adam will respond to your mail. However, if you include a SSAE (stamped self-addressed envelope) he will return an item that you send to be autographed, or if you don't send anything he will probably still send you an autographed picture. Please be patient, as you know he is very busy and he DOES sign everything himself. :) Also keep in mind that he has asked his fans to donate to DonorsChoose in lieu of spending a lot of money on gifts for him. :)