Dar,
Do you want to 'buy,' for example, $300, or do you want to sell, £300?
If you want to say, buy $300, meaning you have a payment to make of $300, you put in the 'buy' space 300 and then in 'Currency to buy' drop down menu put USD and in the 'Currency to Sell' drop down menu, put GBP.
Does that make sense?