There is probably a trim piece on the back wall, a channel with a rubber strip in it. Take the rubber strip out of the channel. Drill a pilot hole through the back wall (in the channel) and into the sidewall. Slightly oversize the hole through the back wall only (slightly larger than the screws your are going to use). Use long screws and screw the back wall to the sidewall. Put the rubber piece back in the channel and your done.
You are putting screws through the frame piece in the corner of the backwall and screwing into the frame piece in the corner of the sidewall. The frame pieces in the corners may be an aluminium section. They are in mine. Or the could be wood 1"X2"s, 2"X2"s., etc. You have to "slightly" oversize the hole in the back wall so the screws don't bite, and instead slips through the back wall and pulls the two walls together. Otherwise the screws would bite into both wall panels and hold the walls apart instead of pulling them together. Start at the bottom where the walls are already (hopefully) together and work your way up to the top. Use washers behind the screw heads in the channel if need be to get a bigger holding area. But everything has to be able to fit behind the rubber piece when you put it back in.