Um, is it plugged in then? If it is a large four-pin molex connector it gets power directly from the supply and should be on.
Generally the other style of fan will be a smaller connector with either 3 pins (sometimes with only two wires) or 4 pins and wires. These would plug into the motherboard or a fan controller. If you are plugging it into the motherboard there are spaces designated for the CPU, SysFan, and Chassis Fan. Either of the last two would be appropriate for the rear fan. If the fan does have 4 pins and wires and you don't have 4 pin connectors on the motherboard, they may not spin.