I know this is an old thread, but I have my spacemouse plus serial and usb working in Windows 7 64-bit and AutoCAD 2011. The trick is to go to the 3dconnexion website and download drivers for the cadman (archived files checked) for a serial spacemouse plus. The cadman is the only archived driver that is 64-bit and for the serial port.
That being said, AutoCAD 2011 has native spacemouse support now, and it is rather slow and laggy with the older spacemouse plus. I still use it because it's better than nothing.