'Seed and Bound' in Mechanica Thermal

Just trying to set up a 'simple' thermal analysis in Mechanica (sorry, Creo 2.0 Simulate).

I want to apply my heat load (and also my convection boundary) to a fairly large collection of surfaces - being the inside (and outside) of a cast gear casing. I'd assumed that I would just be able to use Seed and Bound selection to pick any interior surface and stop my selection at the holes where the shafts stick out - but I can't seem to make that approach work, and both the Boundary and Intent options for selection give me just one option for selection, namely the entire casing.

Any pointers?

Thanks!