I have mapped a key to the fire extinguishers, but for multi-engined aircraft i think you need to select each engine individually and then press the extinguisher key (at least that's the way it was in IL2). That is, unless there's separate keybindings available (extinguisher 1,2,etc listed separately in the commands).
Generally speaking, that's how every "generic" command works with more than one engine (it's the same as the engine start, we select the engines first before pressing "I").
I also seem to remember there's a couple of handy buttons right on the starboard side of your instrument panel, press F10 to enable the mouse cursor (if it's not already on) and just click on it
That being said, i doubt that they would be of much use with a full wing on fire. I think they are contained in the engine nacelles for taking out the fire before it spreads, but if it reaches the wing fuel tanks there's not much left to do.