if that doesnt work, this is what you need to do:
1. uninstall all nvidia drivers
. then shutdown the computer.
2. remove the sli bridge and the 2nd card from the pc.
3. power on the pc and install the latest nvidia drivers, shutdown the pc.
4. reinstall the 2nd card and the sli bridge.
5. power on the pc, it should install the drivers for the 2nd card for you, reboot.
6. try to enable sli.