As stated above, whats your psu? Make sure all connectors are in place, the GTX480 uses 6-pin and 8-pin connectors so therefore for SLI you would require 2 6-pins and 2 8-pins for your configuration to work, also check the settings in the drivers and of course, update if necessary. Good Luck!
I have the Silverstone 1500w power supply. I have used the two eight and six pins. I have enabled sli in software through the Performance gidget from Nvidia. I just cant figure out what is the problem.