Buying or selling to market can be done by clicking the buy market or sell market buttons located at the bottom of the Dom. You can also Flatten all or cancel all your orders from the bottom of the Dom.
The first key information about the Dom is placing trades within the Dom. This can be done by left clicking on either the bid or the ask side. Doing this will place a limit order.
You can also change the quantity of the orders you are placing from the top of the Dom.