The black wire was made hot when it was pigtailed at the outlet. Shove it into the back of the switch. The white wire becomes hot, of course, when the switch is turned on. Finally, screw both control switches into the electrical box. Connecting a dimmer switch is really no different than installing a normal switch.
Most dimmers come with wire leads though, instead of terminals or screws.
Simply wire nut each wire lead to the appropriate wire and stuff it back into the box.