How do I wire my Home Depot combo plug and switch?

How can I add a light switch and convenience outlet to a three way circuit so that the outlet stays hot. I have one switch in the garage and another at the top of the stairs. There are three light fixtures that work with this three way set up. The switch I would like to wire up is at the ‘end of the run’ and has a red, black and white wire entering the box. I wired up the four posts of the combo switch so that the red is top right, black is bottom right (and pigtailed so that it is on the upper left corner for the switch) and bottom left is the white. The lights come on but only very dim and only when the upstairs switch is turned on. The lower switch doesn’t change anything, but the outlet seems to work with the change of the switch at the top of the stairs. On the switch at top of stairs there is a black wire in the lower right, white in top right position and red at top left. Any help is much appreciated, thanks- M.