You need to download the map BSPs and put them in your
steamapps/Momentum Mod Playtest/momentum/maps folder. You can get maps from somewhere like Gamebanana or here:
Rocket/Sticky Jump: http://nnsurf.site.nfoservers.com/Momentum/
The playtest supports CS:S, TF2 and CS:GO maps.
To load ingame, open the console with
or ~ then typemap <mapname>`, where <mapname> is the map you added to your maps folder. These won't be zoned unless you download zones as well, some can be found in the third link above. Hope that helps 🙂