Add you AI aircraft, click on it. Go to the triggered action tab and add the 'start' action (can't remember the category you can find this start action but just look for it).
Very important thing: check the checkbox 'uncontrolled aircraft' otherwise your AI aircraft will start the moment you release the pause.
Then, go to your trigger table in ME. Create a trigger with the condition of your choice. Then in the action column you choose to trigger AI action, and in the drop down menu of this action, choose the one you want (starting with name of unit /coma/ type of action which in your case going to be start).