note: i use this system for NEW clients not me.
I have an OLDDDD voucher and how i do it is act dumb and phone the help line 1800 613 820 , code = LBD
tell them it came in the mail with a bunch of other promos.
The people who answer ALWAYS just give me $75 credit AND set up the account !
WHY? because in about a week you will get a phone call from them trying to sell you a $600+ month deal for them to look after the adwords for you.
anyway all i do is tell them its not a good time to talk and they rarely ever ring me back.
JUST BE careful that you always choose pre-pay otherwise you could end up with a big bill, and they will TRY to get you onto that method.