Just place the base in the editor as you want the base to appear when it is completed by the AI. Then select each unit in this base and press enter. In the menu that comes up there is three tab options, general, ai and textures. Go to ai.
Do the things I said there, unselecting "initially created" and setting the rebuild count to always. The units should appear purple now.
You have to enable the ai to run in order for it to build anything. Don't worry, if you don't make attack waves for the computer player it wont actually attack. Set up a trigger for when you want the computer to build and add these actions:
AI - Start the campaign AI for player 2
AI - Activate bullies in region "Player 2 Base" for player 2
Remember to give the computer (in my example player 2) enough resources and scvs/drones/probes to be able to build anything.
Sorry if I'm unclear, I'm not really good in English. Hope this works :)