We need a lot more information to help here, although the solution will be an easy one.
What version of Archicad?
Was Archicad closed (not running) when you installed the Goodies with the add-on?
When you say that "it says that the add-on is installed" - do you mean that you went to Add-on Manager and found the 3DStudio add-on, clicked on it, and that it said it was installed? At the bottom of that screen, where does it say the interface was installed?
If you're on 26, you're saying that it does not appear under File > Interoperability?
See also, for 26:
and for 25:
Still looking?
