braveheart did give you the website for the Game Engine. Just go to the “Downloads” section and download Blender 2.42a. Blender is much more than a game engine, though. It is a complete 3D modelling program, capable of commercial quality 3D rendering and animation. I’m sorry to say that the Game Engine is not quite on the same “commercial level” as the rest of the program, but it is still really good, and it is very easy to use. If you’re looking for documentation, I recommend that you visit “Download -> Documentation” on the blender3d.org site, as well as the “Info Center -> Using Blender -> Manuals and Books” page. At the Manuals and Books page, scroll all the way to the bottom to find the Game Reference. It is the closest thing to a free book on using the Game Engine that you will find.
I don’t know if you can import DarkBasic models to Blender. In the Blender program, click “File” and bring out the “Import” sub-menu to see all of the file formats that you can import. If DarkBasic can export to any of those formats (for instance, Wavefront OBJ is a common one), then export to that format with DarkBasic and import to Blender. Also, it’s entirely possible that someone has written a script to import DarkBasic models. You could search around the forums a bit (probably in the Game Engine board or the Python and Plugins board) to see if it’s been done.
Hope that helped. Let me know if you have anymore questions.
P.S. - I’m inspired to add a “Getting Started” section to the tutorials on my website…