When did you do the offer? They take time to credit. From the FAQ:
How long does it take to get credit for an instant offer?
Most instant offers will be credited to your account within 10 minutes. However, they may occasionally take several hours or even days to show up. Please read the offer credit information page.
How long does it take to get credit for a non-instant offer?
Most non-instant offers will be credited to your account within a day or two. Occasionally, the advertiser delays reporting for days or even weeks.