Under Windows, hold down the <Ctrl>+<Alt> key combination while you start ArchiCAD until the menu comes up. Under Mac use the <Cmd>+<Opt> key combination to achieve the same. You should see the Special menu to the right of the Window menu. If it doesn't come up you most probably haven't held the keys long enough. In this case you can just restart the program and do it once more.
In AC8.1 MAC OS, this <Cmd>+<Opt> is now a toggle switch. What this means is that if you invoke with Special Menu enabled, you need to use these keys again to toggle it off.
On PC, these <Ctrl>+<Alt> keys must be depressed each time you want to invoke the Special Menus.