I was having problems installing the Agents of Change DLC on my PS3. No matter what I did, I couldn't get the DLC to install.

Then, I remembered: I probably hadn't installed the latest patch for Brink.

If you haven't downloaded the latest patch for Brink and tried to download the Agents of Change DLC, delete the GAME DATA (not Save Data) and delete any corrupted data that may have been left from the AOC install. Corrupted data from the AOC install, if present, will appear beneath any demos or game folders in the Game section of the XMB as a selectable item that says "Corrupted Data" with a torn icon next to it. Delete it as you would anything else on your PS3. Put Brink in your console, download/install the latest patch (which should happen automatically if you're signed in), select an existing character, go the main menu, then quit back out to XMB. I ejected Brink after this process, I don't know if it's necessary or not.

Then sign in, go to the PS Store, download the DLC in the foreground (as someone mentioned in another thread, I never downloaded AOC in the background in any of my attempts). When the install completes, put Brink in your console again, select an existing character, and check the main menu. Everything should appear normally.

This worked for me and I hope it can work for someone else, too.