I also have the QX49. Did you install the cubase script that’s on you Alesis disc?,
if you did the sliders will control the volume of each track and the knobs will control pan, if your vst instrument has midi learn, place your mouse over the control knob in your vst that you want to control ,right click and you will see learn pop up, just turn the knob on the QX 49 that you want to assign it to.