I think you all din follow the correct instruction, I do not have Arduino Maga 2650, but I do know that the driver is different from the previous version where they use FTDI chip as USB To UART bridge. Searching through
Arduino Troubleshooting page, I found this:
On Windows 7 (particularly the 64-bit version), you might need to go into the Device Manager and update the drivers for the Uno or Mega 2560. Just right click on the device (the board should be connected to your computer), and point Windows at the appropriate .inf file again. The .inf is in the drivers/ directory of the Arduino software (not in the FTDI USB Drivers sub-directory of it).
also this:
If you get this error when installing the Uno or Mega 2560 drivers on Windows XP: "The system cannot find the file specified", you might try this suggestion (about adding a "RunOnce" key to "HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion")
might be helpful to look through that page