I found them at the eastern tundra. Since the subdungeon was ice as both the bridge and tundra, i think that's the prerequisite. As stated above survival is important, but there is no need to slay the boss. So you may just rush through several times without holding back on undiscovered tiles until you get them.
If it matters, I've unlocked every other race and class that I'm aware of, and this is my dungeon map so far (right click and view image to see the whole thing):
For me at least the Goblins appeared much later than the Orcs, but maybe that was just random.