The .dcm file isn’t a footprint, it’s part of the symbol (it contains the datasheet and some other metadata, i.e. the DoCuMentation, but that’s not really relevant for your purposes). This is kind of a weird implementation detail that is smoothed over in the upcoming version. You shouldn’t have to worry about the .dcm file at all, kicad should take care of it automatically.
Footprint files end in .kicad_mod, and they are contained in libraries which are folders that end in .pretty. The libraries (.pretty) are what you want to add to the footprint library table.
This FAQ may be helpful: How to get a downloaded symbol, footprint or full library into KiCad version 5?