Another way:
1)run REGEDIT.exe;
2)find the key named as HKEY_CURRENT_USER\Software\Graphisoft\ArchiCAD\ArchiCAD 8.1.0 INT R1\Special Menu, nad change the value to 1.
ArchiCAD 8.1.0 INT R1 may be not correct. it is according to which version of archicad u have installed.