If your wiring is not 18/5 and there is no blue wire, then you would either need to reuse the existing fan (likely green) wire or run an extra wire between your HVAC and the thermostat to serve as the C wire.
To reuse the fan G wire, you simply move it to the C terminal instead of the G terminal on the thermostat and also move it to the C terminal on the HVAC control panel. You can recognize the C terminal on the control panel by viewing step 5 of the easy installation video.
iotGizmo can also assist you on the toll free support line. The implication of this approach is that you cannot control the fan separately at the thermostat and it will go on/off along with the cooling as well as the heating if using forced air. This is probably a good trade off to enable use of smart energy as very few people ever try to run their fan independently of heating and cooling. Certainly easier than running an extra C wire per the approach below.
To add an extra wire to serve as the C wire, it would need to be connected to both the thermostat and the HVAC control panel so that continuous 24V AC power is flowing. The degree to which you have access to run a wire from each end, such as the area behind the wall that the thermostat is mounted on, will determine the level of difficulty whether as a DIY or a service call from a professional HVAC technician.