AFAIK: The only way to get 3ds into the library is with an add-on that only works in ArchiCAD 7.0. It is called 3ds_in. It can be found in the Goodies folder (as I recall) and needs to be moved to: >Applications>ArchiCAD 7.0> Add-Ons>Import-Export
Once loaded, it is accessible in a menu which I think is called "Extras". I don't remember for sure and I don't have AC7 loaded right now.
This functionality should be available as a standard feature in File > GDL Objects > Open Object... Is this already in the wish list? If not, it should be. Even better would be drag and drop 3ds parts straight from the web.