After cloning, you need to clone the submodules too.
./setup.sh
Check the manual of your git clients for instruction how to do that.
./rebuild.sh
Use Visual Studio or install make and mono and use the instructions for Linux.
After building everything using make or Visual Studio, you should launch the OpenRA.exe with the arguments Engine.ModSearchPaths=./mods,../mods Engine.DefaultMod=ra2a Game.Mod=ra2a