To hook up 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 produce consistent results, make sure you're shooting in Manual Mode on 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 the camera ISO to get good exposure.
Canon Cameras
Then go to your Canon camera settings, and make sure your Flash Control and Flash Firing are set to Enable.