The built in collision commands are slow and inefficient, and sometimes incorrect. The first thing you'll want to do is make sure you can get your model into on of these formats; .3ds, .x, .dbo (listed in order from least to best possibility).
Once you've got the model in that format, importing into darkbasic is as simple as using the LOAD OBJECT command with the file name of the model, along with an object number for the file.
After that, seeing as like I said using the built in collision commands is a bad idea, do a search for collision and check out some of the more popular threads. Sparky's collision dll is a free plugin for dark basic/dark basic pro that allows fast, efficient, polygonal collision detection, with features like sliding collision as well.