First of all, OP and EAW use the same model format. You can also use SFC I models. They will not have illumination so they will look a little bit dark, but they will work just fine. To use the model, you need to put the file path in the Geometry field in the shiplist, beginning with "assets", i.e.: assets/models/agab/gabi.mod. There are two shiplists, one in Assets and one in MetaAssets. Both have to be identical for the game to work properly, though in skirmish mode you can use only the Assets shiplist. The easiest way to edit the shiplist is to use shipedit. Then you can just hit the browse button next to the geometry field and find the appropriate model. I am not sure where you can get shipedit now; the last I checked Cleeve's Staryards was down, and I think Eagle Eye is down by now.