I had the same prob, I searched for some script that alows blender to save models in hl mdl format, but the answer there is no such script. So if you want to make models for hl with blender you need an aditional modeling program that can save to mdl. format. I suggest you to use milkshape it isnt profesional modeling program, but it can save and open models from almost any game and program you could need.