I'm not familiar with your camera and flash but I'm a fairly keen photographer. It sounds like your picture is being over exposed, which could be for a number of reasons.
You'll probably want to make sure your flash is being used in TTL mode, you also need to make sure that the camera is adjusting it's exposure to take into account the burst of flash, otherwise it's shooting the picture as it thinks is normal but the flash is firing too which will make it over exposed. To do This, try taking a picture witout the flash, then take a picture with the flash on and in TTL mode. Look at the settings for each picture and they should be different on each one.
Lastly, depending on your lens' widest aperture you may want to stop down some more; with flash photography a good rule of thumb is to assume that aperture controls the strength of how much flash is received (so at f8 the flash would have less of an impact on the scene than f2.8) while shutter speed and ISO control the amount of ambient light. Your camera and flash will have a maximum sync speed that could be around 1/200 (so you won't be able to shoot faster than this and still have the flash work as normal), but going slower will allow more ambient light to balance the flash.
That's a bit of an info rant but hopefully it will help, of course feel free to ask if you have any questions though. It' very possible that there's an easy solution to your problem that someone with the same gear might be able to solve, but that' the basics otherwise of what could be going wrong.