The PS3 version of Mass Effect 2 says it comes with a interactive back-story by Dark Horse comics.
How do I access this content? There doesn't seem to be anywhere in the menu to get to it before starting a game.
Firstly you will need to download it (and the other DLC) using either the in-game menus or directly from the Playstation Store. The game comes with a code that you can use to redeem the downloads (as long as you don't have a used copy of the game).
The comic actually forms part of the game (including a few points where you get to choose what happens), so all you need to do to see it is to play through the game's introduction and the comic will start automatically.
Given that the start of the comic flows on from the introduction, and the interactive element, it wouldn't make much sense to have it viewable before you have started the game.