You can't get Ascension on the PS3 version of Call of Duty: Black Ops. Ascension was released as part of the "Anniversary" DLC pack, which was not released for the PlayStation 3 platform.
The "Anniversary" DLC pack included:
* Ascension
* Call of the Dead
* Shangri-La
* Moon
Unfortunately, if you are playing on PS3, you won't have access to these maps. You can only access the base maps that came with the original game.