If Boxshot doesn't let you 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 can upgrade to the Ultimate edition by just paying the difference here: https://boxshot.com/boxshot/order/renew/
If you have Ultimate edition and Boxshot still doesn't let you to drag a file, it is probably an unsupported format. Try 3DS, DAE, OBJ or FBX which are definitely supported.
If you are sure, there is an error here, please contact support to get this solved.
Customer support service by UserEcho