You buy using the green button, where it says 'price' you put in the price you want to pay, I paid 0.00005002 BTC and 150 in the amount box
it wasn't taken at first so the offer goes into 'buy orders' until the price drops and somebody comes along to sell.
or you can take the price in the 'sell orders' columns (pick the lowest price) which will generate a sale straight away but make sure you check the current going rate first to give you a rough idea what the price should be, I use blockfolio or coinmarketcap to check what the current prices are.