Did you use the clean installer to install them in the game? That tool can be found at the MTS2 site.
After unzipping the files, did the houses showed as .sims2pack or .package files?
House you install should normally appear in the lot bin folder once you load the game.
If some cc came with those house, you might also be in need of the CEP program that can also be found at MTS2 site.
And to finish, here is a sticky post from this section that should be also useful to read:
Dr. Know-It-All's Diatribe No.2. - Installing Downloads.