Right, so what you need to do is first you make sure that in the poly-to-subd option box, the max poly limit is more then the number of polys in your polygon model. That way it might not even convert.
But I think I know what is going on... I had the same problem just this morning. The divisions are all quads, so the SUB-D wont proccess the vertex information. TO fix this you select a vertice that DOES show up and move it VERY SLIGHTLY. The field grows, and you keep on doing this untl you get to the region you want to edit.
To do this you also may want to set the vertex display level to 0 or 1, not the default 2. YOu right click and click coarser. When you want to go back to 2, you use Finer.
Also, to get the actual polygon model to affect the SUB-D you right click and use Polygon. THen you use all of the normal polygon model techniques and tools.

Hope that helps
Cheers,
,Matt