You need to run the AI script "BW Terran 2 - B (Fill Silos with Nuke (Type 2))."
Don't use Type 1. It's problematic, to say the least.
Set the nuclear missile to 0 mineral cost, or make sure the computer has enough resources to make one, because they will actually be building them.
Set the build time to 0 if you want them to be instantaneous.
Finally, make sure the computer has enough supply to make one. Stack 25 Supply Depots on top of each other somewhere, if you want. Just make sure the computer has the necessary supply available or it won't make them (each nuclear missile uses 8 supply, so 1 per supply depot).