Corky R
If you just bought this unit, there should be a user manual with it that should detail the location of all the controls. If not, I'd sure contact the store you bought it from right away.

William B
I have a 18,000 the vent button is in the cold air vent, grill mine is just a little knob i pulll out or push in,

Thor
My LG 8000 btu unit doesn't have one.

Breath on the wind
All window air conditioning units have an option to vent in outside (warm but fresh) air and mix it with the inside air that is being cooled by the unit. Closing the "vent" means that the air coming from the unit will be cooler because it is not being warmed by the outside air. The trade off is that you are not adding any fresh air to your room. In practice, most people would not notice the absence of the fresh air and the unit is more efficient with the fresh air vent closed. Check with the owner's manual if you bought it new or on-line (you will need to know the model number) if used to find the operating mechanism to close the vent.