You go to the folder that stores AIs for your switcher, pick an AI you would like to edit, right click the AI file (not the notepad one) and click open with. After that open it with notepad. Increasing or decreasing the weight makes the AI more or less likely to build that unit. Increase or decreasing the limit decides the max ammout of what unit the AI will make. Always remember to Save As a new file and not to just save or you will lose the original.
Note you have to do this for every race. The only races there are the basic 5. I'm not entirely sure how they effect 3rd party races. My guess is either they always use default or it effects the race equivalent.
I hope that I have helped you.
If anyone else has anything to add or change feel free.