Directly import MDL into Blender? Never heard about that... If i wanna import MDL Files into Blender (what isnt possible in my opinion), i import it into milkshape, export the MDL file as .obj, import into Blender, do what ever i want to do, export as FBX.
Tutorials: [Blender]Terrain creation ENG/GER [Blender]Low Poly Tree Modeling [GIMP]Create a Texture for Terrains CLICK HERE