Hi Ethan,
Was this plugin done for TG2? Sounds like it was done with TG v0.9. TG2 doesn't use TGW files at all, there is no way to do so. You can use TER files, actually can't think how or if you save them from TG2 right now, I'll take Calico's word for it :-), but you can certainly use existing ones. However that's not much use to you if you're using procedural terrains, which are saved in the TGD file.
There is a file detailing what's taken on the command line for TG2 in the Documentation folder in the download.
Regards,
Jo