Weird, I thought I replied to you in PM, but I can't find the reply.
Anyway, here it goes again:
You probably cannot do this easily in mod tool since its designed primarily for basic model building and not advanced physics simulation and animation like the regular XSI.
Within regular XSI, you can select your model, then go to NetView (Alt-5 or View->General->NetView), where you can find, under the "Model" tab, an operator named "Shatter". Adjust the number of fragments, crack detail, randomization of fracturing (for best "shatter" effect, check both uniform and pseudoRandom options) etc.
After that, you have to set up your simulation. You have to convert all the fragments into dynamic rigid bodies, adjust their collision so they don't immediately fly apart, and then set up a physics simulation of the blast. I'd use an Attractor daemon with a large positive value (positive values repel instead of attract) and animate its range so that you get a "shockwave" effect shattering buildings in succession.
As I said, this probably cannot be done at all in the mod tool. You could try to do it by hand by using a simple grid object with deformed surface which you can then rotate at various angles and use to "cut up" your model into fragments, but that would probably be way too time consuming, especially when you have multiple buildings.
Also note that physics simulations tend to take a lot of computing power, and the more objects you have flying around, the more time it will take to calculate everything, nevermind the actual rendering. Holywood-type cataclysm effects where you see whole cities blown up tend to take a lot of time to do even with their dedicated render farms, so keep that in mind.