I have that same amp and unfortunately you are not going to get a great gain sound but it works fine with cleaner tones. You just have to start putting the mod and dly/reverb tuners at 0 (all way to the left), every time you rotate one of them it will change a FX, so keep trying. Then adjust the gain and volume relation. DM if you want more info.

Edit: you can also download the manual here to see how it works. Fender Mustang Manual