Ceiling fan grounding
The metal box should be grounded, unless your wiring was done incorrectly. Look in the back of the box and you should see a cable coming in to the box, with black, white, (possibly red), and a green or bare copper wire. The copper wire should be screwed into the box itself, providing a ground. 

If not, it's possible the cable is armored or run through metallic conduit, which would provide the ground.

In either case, simply attach the green wire from the fan to the metal box, being sure to strip enough insulation.