You have a simple setup with R (24V power) and W1 (HEAT). The fan is turned on automatically, along with HEAT, when the thermostat applies the power from R to the W1 connection.
The FAN terminal is labelled G and you need to connect that to the G terminal on your thermostat. Then, provided you have a FAN ONLY setting on your thermostat, it can control the fan independently.
You're going to have to run a "G" wire though since the currently line appears to only have 2 wires.