I have two sets of cables in the wall box. They each have one white wire and one black wire. They are not marked. Only one of the black/white wires that come from one of the cables has power (120volts) the black and white wires from the other cable don't measure any voltage. The thermostat I bought has two black wires that are labeled as line 1 and line 2 and two red wires labeled as load 1 and load 2. given this information, how do I connect the wiring properly.

The thermostat you have is a double pole for 240 volts, you need a single pole thermostat for 120 volts.The hot black will connect to the LINE and the remaining black will connect to LOAD, the whites will be spliced.