I've never heard of that flash, but I'm guessing it's a manual only flash. So, you put your camera in auto, and it tries to determine what the proper exposure is (without the flash), takes the shot and fires the flash which only adds to the exposure. How much you over expose depends on the power level you choose.
Using a manual flash with any sort of auto setting on the camera is difficult and takes understanding of what the camera is trying to do and how it's exposing. Even then, there's going to be a certain amount of guess work and experience. If you want to do it this way I'd recommend moving your exposure compensation down two stops. So the camera tries to underexpose the scene by two stops, and then dial up the flash power until the exposure looks decent. Depending on what the flash hits the exposure is going to vary. Just keep in mind that the camera will have no idea what the proper exposure is, because it has no idea how much light the flash adds to the scene. The only way is to check the image afterwards and adjust accordingly.
I do this a fair bit with my camera in manual and an off-camera flash in "dummy mode" shooting straight into the ceiling. I'm constantly checking my results and adjusting, but I find it to work quite well. But by getting my flash off camera I can get consistent results from it. So in essence it just becomes another ambient light.
