The version of the game distributed to users is archived.
My knowledge may be a little out of date but I think the steps to do what you want are:
(1) Go to the directory containing Zero-K.exe (we will call this DIR)
(2) Create a file "devmode.txt" and put it in DIR. You can leave it empty. This enables step 6.
(3) Open or create a DIR\games\ subfolder
(4) Pull or download the contents ofhttps://github.com/ZeroK-RTS/Zero-K
into a subfolder DIR\games\Zero-K.sdd\
(5) Make changes in the DIR\games\Zero-K.sdd\units\ and/or DIR\games\Zero-K.sdd\scripts\ folders
(6) Start Zero-K and go to Settings -> Developer -> Singleplayer (at the bottom) -> Zero-K Dev
(7) Play single-player game.
Do be aware that by messing around with your main Zero-K Steam install, particularly if you use many of the options in the Settings->Developer tab, you are in some sense "breaking warranty" and we may not be able to help you with further technical issues. If you download the lobby from Github as wellhttps://github.com/ZeroK-RTS/Chobby
and mess around in there then your main install is safe. I think the procedure is mostly the same except the executable is called Chobby.exe, not Zero-K.exe.