The DarkMatter models were made for use in DB; any .x or .3ds object file can be loaded into DB, you can even make your own
Do what INH said and DB will load the object into the program and you'll see it on screen, take a look at the object commands in the help file to see what you can do with it.
LOAD OBJECT "example.x",1
Would load a model called "example" from the project folder into the program, and DB would refer to that object as object 1.
If you copy the object file into the project folder (same folder as your program) you don't have to type the full path.
LOAD OBJECT "buildings/house01.x",2
This would load an object from a sub-folder called buildings.
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