There is nothing to download there- you asked how do you make a B3D file and that site I just gave you will tell you all about how to make one.
To make a B3D file all you need is notepad, the knowledge of how to save a file in B3D format and Structure Viewer to see what you have accomplished. Notepad can be found in Start --> All Programs --> Accessories; you can save in B3D format by selecting "All Files (*.*)" in the box under where the filename is typed labeled "Save as type" and adding .b3d to the end of your filename; Structure viewer can be downloaded from the official BVE website at mackoy.cool.ne.jp (to view an object drag it in from windows explorer to structure viewer)