The problem must be with your Add16.
You can confirm this by renaming your Add16.hdl to something like Add16.hdl.x to force the simulator to use its built-in Add16.
One other thing to check, since you didn't indicate what line is failing to compare. Double check that you are loading the Inc16.tst file form the same directory where your Inc16.hdl is. I've made that mistake way too many times of the years :-(