I don't know exactly what to search up for this so I'll make this thread/
I have started to create a map (not a big project) and I have added custom units that I've created. In documentation it says something about putting dc: somewhere. My question is where exactly do you type that?

Right before the side. For instance, for a custom Rep side:

ReadDataFile("DC:SIDE\\rep.lvl",

The DC: tells the game to load from data***/sides/rep.

Check out the FAQ thread.