Jasna,
What type of object did you download? In order to be read directly by ArchiCAD the objects must be a .gsm. If you have something else (i.e. a .skp or a .obj, etc.) then it might be able to be converted to a .gsm to be used by archicad.
If the objects are the correct type already, then place them in a folder/directory. Open archicad and load that folder/directory into the project via the library manager.
Erich
AC 19 6006 & AC 20
Mac OS 10.11.5
15" Retina MacBook Pro 2.6
27" iMac Retina 5K