OK, there are a few ways to have the AI build in CnC
1) Skirmish Only > Build > Build Building
This option will allow you to build any building of theirs, granted they have the ability and prereq's for, but that building must also appear in the AIData.ini file (I can explain that too if you wish). So for every instance that the AI is told to build that building in the scripts there must be another listing of it in the AIData.ini file.
Also in that drop down youll see build on flank and build on front for both any type of building and defensive structures which default as gatlin cannons or patriot missiles. For those options though the building does not have to be in the AIData.ini file, becuase you are telling them right there where to build it.
2) Player > AI > Build near a supply source
This option is the one I use to make the computer player expand. Buildings here do not have to be in the AIData.ini file because again you are telling them where to put right in the script. The computer will build it near a supply pile with atleast xx amount of cash remaining. This one is real handy becuase you can tell the computer, 'if you have 2 supply depots, build a third airfield, 2 more drop zones and 3 patriot missles, and if you have a third supply depot do it all again' and so on. Just be careful how you script anything in this part becuase its easy to accidently make the computer keep building buildings right next to the supplies and building further out with no control and it can be kinda awkward and slow down the game with lots of unnecessary buildings so I try to keep most my scripts specific to how many supply depots he has becuase that typically correlates with how much building space he has as well.
Turtling is not a strategy, its a weakness