I don't have that mod, but my guess is (from having disassembled quite a few chinese mods) that it's not a simple matter of sliding it back in. It likely fell out because a little piece of plastic on the button itself, or a piece of plastic on the inside of the mod, broke and therefore the button was able to fall out. If this is the case, there is very little you can do about it. However, your best bet is to open them mod and take a look inside. Perhaps you will be able to get it installed again from the inside (which is usually the way buttons are installed, from the inside).
Of course, I could be wrong and that all you have to do is slide it back in somehow, but that would be a really dumb way to design a fire button (so that it can easily fall out).
This person dropped they Dynamo and, among other things, found that the fire button came loose due to a crack in the housing. So if you dropped you Dynamo, that's likely you problem, or something similar.
Dynamo fixable?