The prizes vary (depends on when you renew and what they're offering at the time).
The prizes will be active when Apple sends you an eMail confirming it, probably immediately after you renew. Note: the prizes do expire, so if you choose a cash discount you better be planning to buy something.
To get the bonus, you don't have to renew at the .Mac site; you can buy a retail copy or whatever; you just enter a code to activate it.
I suppose it's possible you don't get such notices if your .Mac preferences were set up so that you don't get the .Mac Connection newsletter or some other correspondence from Apple.
In my case, I used two discounts: I bought Macromedia Contribute for $99 which comes with a one-year .Mac account (ordered from Macromedia site, not Apple's) and then took the $30 from .Mac renewal and bought Panther.