To hookup an AC-powered flash strobe to your camera you'll need a PC Sync socket. Higher-end cameras have a PC Sync socket where you can plug in your AC Flash Strobe. For lower-end cameras such as the Nikon D90 or EOS 6D, you’ll need a PC Sync adapter.
For the external flash to work and to get consistent results, make sure you're shooting in Manual Mode in the camera. Set your aperture to F7 and shutter speed to 1/100. Set your flash power to 3/4 of the way then test and adjust camera ISO to get good exposure.
Then go to your Canon's camera settings, and make sure your Flash Control, Flash Firing is set to Enable.