I have an Intertherm model MMHB-070ABHR gas furnace.

I am replacing the old mechanical thermostat which did not have a fan on switch with a digital thermostat.

The wiring to the thermostat now has only 2 wires. Which wire on the furnace do I wire to so that I can use the fan control on the thermostat?

The unused wires on the furnace are green and yellow.

In order to use the FAN switch on your thermostat, you'll need at least 3 conductors. The third wire will be needed to connect the G terminal on your thermostat to the fan relay.