Trouble wiring a Leviton DZ15S (originally a single pole with two black?)
On this switch, black is line. That is the black wire coming in from the cable on the right to the black terminal block. Red is load. That is the other black wire on the old switch, which goes to the fan. Neutral is the white line coming in from the right.

Move the existing wires from the old switch over to the new switch. Add a white jumper (a short piece of wire, maybe 6") from the block terminal holding the other whites to the white terminal on the switch.