If Boxshot doesn't allow you to drag a 3d file, please check the edition you have - it is in the title of the window. If you see Home or Professional there, you can't load external files. Only the Ultimate edition can load them.
You may upgrade to the Ultimate edition if you like by paying the difference. Please contact the support to do so.
If you have Ultimate edition and Boxshot still doesn't allow you to drag a file, it is probably an unsupported format. Try 3DS, DAE or FBX which are definitely supported.
If you are sure, there is an error here, please contact support to get this solved.