Are you using an EV controller? What settings do you have it on - preflash or not?
What camera setting are you using? e.g. Aperture priority mode, Timing priority mode, manual, etc?
The best way to test this is to take a picture of a mirror in your house. If the strobe is firing on the correct flash the picture should be washed out - you'll clearly see the external strobe firing. (your camera probably 'pre-flashes' in either A or T modes, too fast for you to see, so the strobe needs to be set to not fire on the 'pre-flash').
It's also a good idea to 'tape over' the internal flash with something like electrical tape on the exterior of the housing, to block most of the forward light which might cause backscatter (and which is why you wanted an external strobe in the first place). Assuming you're using an EV controller it should still be able to see enough of the strobe flash from the side, around the tape, to fire the DS51.
