You have to press F11 in the game to switch to the new FOV after applying the Game Optimizer setting. In some cases additional steps like this one are unavoidable. If that is the case, the additional steps are mentioned in the according optimizer entry.
If applying the FOV automatically fails for some reason, you can also do these tweaks manually in many cases by googling for “FOV + Game name”. For Mirrors Edge you can find multiple tutorials, one of thhem here: http://steamcommunity.com/sharedfiles/filedetails/?id=138177233
Alternatively you can use the 3D FOV Enhancement in the vorpX ingame menu. Directly setting the FOV in the game is usually better though.