First, put the file you want to use in your program in the folder...
"C:\Program Files\DarkBasic Software\Dark Basic\My Projects\[Your Project's Name]"...(I might have mispelled something)
Then, Type in the following code before your main loop...
Load Object "[File Name With Extension]",[Object Number]
Like if your Object was named CoolCar.3ds and you wanted the obect to be object one then your code would say...
Load Object "CoolCar.3ds",1
...Well...I hope I helped...
[Edit]
You might have to export the 3D Model as a File Type that DarkBasic Classic can handle, .x or .3ds ect...
Thou shalt not thwart the way of the dragon. For thou tasteth like chicken.