You need to create a "Location" which is done by going to the Layers (?) menu and choosing Location. Then drag a box around the general area of their base. You can then name this location whatever you want (like "Red Terran Base")
Then you will have the choice to use this Location when you run the AI script. It won't work with "Anywhere"
Also, you need to play a "Use Map Settings" type game. You can use melee, but the only thing that will be used is the starting location(s). You shouldn't need any AI scripts in this case. Just make sure there are some minerals and maybe gas nearby, and enough open space to make buildings. Make sure the nearby terrain is buildable.
Locations have other uses too, when you get into trigger maps.