It is hard to tell from your post whether you are having a problem with the DP-15 or the LED RGB strip, or both.
First a side note: Jeffery gave you the official (and surely the safest) advice not to plug an LED fixture into the DP-415. I've seen that recommendation on this forum, but I saw it after I had been controlling LED fixtures with my DP-415s for a long time. [So I still do it, and have had no problems.] I ALWAYS, however, put the DP-415 into "switch mode." Even in that mode the power supplied by the DP-415 is not as clean as the power from the wall, but it has been good enough for my LED fixtures. Proceed at your own risk.
While you can dim a light bulb with a DP-415, you absolutely cannot dim an LED fixture with the DP-415. All you can do is turn the LED fixture on or off (supply power to the socket on the DP-415 or turn the power off to that socket).
I can't make any sense of your statement that the LED is really a 12V unit. If you somehow plugged a 12V unit into a 120V A/C wall socket, I can't understand how you can say that the LED "works fine." I'll proceed on the assumption that your LED fixture is a standard 120V fixture.
Test whether the DP-415 is working by plugging a regular house lamp into its outlets. If you have the foot controller for the DP-415 use it to control the light (power to the various sockets). If that works, see if you can get your computer to control the lamp. [I don't know anything about the Enntec USB Open except that it looks like you've got to program it as well as set up your DMX program, i.e., it looks very complicated.)
If you are trying to control an LED with a DP-415, then you must not be using the LED in DMX mode. So, was the LED doing what you wanted it to do (when plugged into the wall socket) before you tried controlling it with the DP-415? If so, then you probably fried the LED by plugging it into the DP-415 when the DP-415 was not in switch mode (you sent the LED too little current).
If you've never gotten the LED to do what you want it to do when the DP-415 gives it power, read the manual for the LED and figure out how to get it to do what you want it to do when you plug it in.
Then, and only then, should you expect the computer program to talk to the DP-415 and tell it to turn on the power to the LED to get it to do what you want it to do.