Your stance should be about shoulder width, and your knees should be flexed slightly. If you are falling off the step, you need to angle your skates more perpendicularly along the step. If your skates are riding up and over the curb, you need to angle your skates more, or lean backwards a bit more. Be prepared for the shock of the ride so you won't fall. Also, there are alternative ways of getting up onto the curb.
Some people like to 'step up' onto the curb.
This would entail putting one foot up there, beginning to grind, then pull the other foot up.