Hover over the grey box in the top right corner to reveal the menu. Select the team you want to place units for by clicking on the team's color, then select what type of unit you want to place. Click on the backdrop to place that unit. NOTE: The AI changes right now will only be applied to Red units until they are complete.
Credits: Me, for the art and coding, and Landfall Games, for making this game's inspiration: Totally Accurate Battle Simulator.