I've had that happen too, so here's another possible solution to that same problem.
Step one is to follow The Dude's directions for resetting Stuffits preferences.
If that doesn't work then make sure the UT CD, not an .smi or disk image, is mounted. To cover all bases, place the update installer in the UT folder and run it from there.
Everytime I've reinstalled UT and had an error while updating, that was it—UT had to be installed all over and the update performed properly.