Instructions
Click the "Scenario" menu and select "Forces." Place the computer player on the same force as the human player.
Open the "Triggers" dialog and create a new trigger. Set "Player" to the computer-controlled ally and set "Conditions" to always.
Set "Actions" to "Set Player [x] to Enemy" where [x] refers to the enemy player. For example, create two actions if players six and seven are meant to be computer-controlled enemies: "Set Player 6 to Enemy" and "Set Player 7 to Enemy."
Create a Location box inside the computer-controlled ally's base.
Add a new trigger for each computer player, both ally and enemy. Pick "Always" for conditions and "Run AI Script at Location" for actions. Select the location you created for your ally's base.
Pick the AI script you wish to use for your ally. "Insane" scripts are very powerful and effective. "Campaign" scripts are generally more passive and less effective. Experiment with different AIs until you find a difficulty setting that fits your map.