You need to change the angles ? it'll push horizontally by default. Try putting -90 0 0 for Pitch Yaw Roll (match the numbers to the order of the PYR in Hammer, I think that's in a strange order).
Also, you might need to have the player fall slightly in order to be pushed vertically, i.e. having the area on which they walk to get pushed one unit lower than the bit they were on before, and then putting a one unit func_illusionary on top to make the floor look flat.