I'm having a very similar problem. I'm using PlayTheWorld, and I'm just now getting into scenarios & units for Civ III. I did a bit of that in Civ II, but it's rather different this time around.
Anyway, I'm trying to add in one of the dinosaurs that came in the extras section. So, I copied the directory from Extras to the units folder in PTW, and I've double checked several times to make sure that the unit/folder/config file's names all match. Still, as soon as one is built (or put in the game, I've tried starting with one in play) the game freezes.
What other errors might cause this?