You need to download the addon from the abvent website, or get it from the Artlantis Studio CD.
http://www.abvent.com/support/download/
It's called
Add-on export from ArchiCAD 10 to Artlantis R-Studio.
Make sure you read the attached
Readme file because you have to copy the APX file into the Addons / Import Export folder, and the DLL file into the main ArchiCAD folder.