OK, got it.
Thanks Djordje, I could not uninstall the driver manually because the installer had already removed it. However, I tried shutting down the computer (versus merely restarting) and rebooted. Then tried the Amyuni installer and it worked.
ArchiCAD 12; Artlantis Studio 2
MacBook Pro 2.4 Core2Duo, 2GB, OSX(10.5) and XPpro(SP3)