This is a Steam issue and you can fix it in most cases by waiting a few minutes or closing your Steam client completely, then restarting it.

If the game for your accepted key does not appear after restarting the Steam client, please contact Steam support as only they can help you with that.

Also, ensure you have updated to the latest version of Steam as this occasionally fixes issues. 

Please also note that some keys install additional downloadable content for games and don't themselves produce new entries in your Steam game library.