I just did mine, you just go to 'Browse Games' in steam. Then select either gold or silver. On the purchase screen that pops up you will see a 49.95 credit for you ati voucher.
Obviously you must have already added the ati serial number under "Register Games I already Own" as im sure most people did months ago to be able to play the beta.
I get the ATI Coupon -49.95,- so the total is 10.00,- on my Silver package, but when I press the "Purchase now" it changes to 59.95,-
I got Condition Zero with my x800 card, so I'm wondering if I'm actually getting that offer or I'll have to pay 59.95, if anyone know I'd appreciate it.