Dimmer switches. I have a basic wall slide dimmer switch with 3 places to put 2 wires. 2 screws on

Question

I have a basic wall slide dimmer switch with 3 places to put 2 wires. 2 screws on the bottom & 1 on top next to the ground wire. I hooked up the 2 on the bottom- nothin. I reversed the wires, still nothin. would the one on top be a + or --. which of the bottom 2 would I use for the other wire.