Generally, if your block has parallel sides you can just use to push a hole through the geometry. However, if the sides arent parallel youd have to extrude the shape you want to cut through the slab then move it in to position, select the whole thing->right click->intersect with selected. As you said, all this needs to happen with ungrouped geometry (or geometry all in the same group) for it to work. Itd be useful if you could post a model showing your problem btw, hard to say exactly what the problem is without an example.

Have you grouped your floors?

Hi Kim, Listen to what Honoluludesktop (OMG - calling someone like that ) suggests; there are some good points there. Also, just start doing it; jump in (but be prepared to having to start over) and whenever you encounter some problems, come and ask. The best way of learning SU is to practice.