Set the thermostat well above or below the current temp using either heat or cool mode. Then try the "ON" for the thermostat. If this doesn't work then it might be that they didn't wire in the fan.
I can tell you more if you can answer the following:
How many wires are attached to the back of the thermostat(3,4,5)?
What colors are the wired attached to the back of the thermostat (red, green, white, yellow, black)?
What terminals are they attached on the back of the thermostat (R,C,Y,W,Y1,W2,G, etc...)?
Same information as above but, this time we want to look inside the air handler where the thermostat wires connect.