I'm sorry you did not get your credits right after your download, however, it is not unusual to experience a slight delay between the download and the crediting.
Just be aware that the credits can be gained for apps you download for the first time only! Some offers require more than just the download, so read carefully the instructions.
If you have followed the instructions, you can see the status of your offer : go to "Earn Credits", "Complete offers to earn free credits" and tap the "Support" button at the footer of the page. From there, please report all offers that have not credited you!