Nevermind - the Darkmatter models CANNOT be dropped into T3DGM. Trying to add them craps out the program no matter if you try to use them as players, bullets, items or anything else.
HOWEVER I found that if you have recently purchased T3DGM then it apparently already comes with all the Darkmatter 1 models preparred for use in T3DGM (possibly some of Darkmatter 2 as well though I'm not sure). They are in the USER section of T3DGM. Scroll past horror, wa, etc all the way to the right where it says USER and WEB.
Also you can go into windows explorer and copy/paste most of the models from one use to another. For example I put a tank into the bullets folder and my AK-47 was tossing out tiny tanks as bullets
Be careful however as the player and enemy models require extra data which is not obviously included in the items, bullets, flak, etc. so more than likely you couldn't have an RPG round as a player or an enemy though you could use it as an item or obstacle.
The Darkmatter models appear to lack some kind of .dat file sthat T3DGM is looking for... or there is some other intrisic problem with using them as is. I only preformed a few quick tests all of which failed