On the trading ladder (DOM), go to the price at which you would like to buy or sell the futures contract and left click on the bid column or offer column each side of the price.
For example in the image below the trader clicked where the '14' is shown to add his 1 lot to the bid queue at the price of 3281.75. (To cancel the order hover over the order cell showing '1' below and an X will appear, click that to remove the order.
You can see a demonstration of this in this video.