You need to download Shaper, (use the trial wisely) to open .DTS files.
Dts format is pretty complicated and it seems to be designed to not be edited by anyone else except those with the original model.
And the G36C you want is pretty much in Sgt walters weapon pack, that is if you are fine with hands and reloading.
And you can use blender(free, many more features), milkshape(simple easy to learn), wings3D(have to export model through either blender or milkshape), or sketchup(same as wings 3D)
I prefer blender