1) Make a folder anywhere, and put a B3D file and it's textures inside it. (I think it takes OBJ as well, but don't hold me to that)
2) Go to "Program Files (x86)/3D World Studio" and open "ConvertModels.exe". Navigate to the folder you made, and click Ok. Then click cancel, as it won't tell you it's done it or not.
3) Go to your folder and you'll find an SMF file.
4) Put the SMF in "Models/SCP" and put the textures in "Materials/SCP"
Then you can place it in your map in 3D World Studio

NOTE
If the scale is wrong, select the model, go to Edit -> Properties -> Matrix and change the scale-values.