This is a typical problem on the very last mission, but it happens in other missions as well. Be sure you have HUGE ROOMS to draw in your creatures. The last mission requires super-large rooms. If you have a captured portal and big enough rooms, those monsters will come.

General Hint Pages
FAQ: Why don't I get monsters?
Hints, Tips & Techniques SET 1
Hints, Tips & Techniques SET 2
Hints, Tips & Techniques SET 3
Hints, Tips & Techniques SET 4

Dungeon Keeper Walkthrough