Once you have logged into your wallet, click on the Trade title.
Click on Advanced. You can now copy and paste in the orderbook pair you wish to trade for or you can click on Add Pair to bring up a menu to fill out to add a new trading pair to the client.
rVSB2uzC9cjKfoMQdpQW78qUdSkdHiU4y is the Streets of Silver Ripple address which issuers the Ripple IOUs to its clients. In this example, we want to trade Canadian Dollars for US Dollars.
Once you click on Add, you will be brought to this screen where you can post a bid for USD.
We are instead going to click on Flip and fill out and order where we want to sell our Canadian Dollars for US Dollars (an Ask).
You will be asked by the client to confirm your order.
Wait 3-5 seconds and you will receive a success message in your client.
Here you can see your order in the orderbook and an option to cancel it.
If you ever wish to cancel your order, the client will ask you to confirm your choice.
Now lets say you want to purchase XRP with your Canadian Dollar IOUs. You would add a pair for XRP/CAD.rVSB2uzC9cjKfoMQdpQW78qUdSkdHiU4y. You can then place a bid for how much XRP you want to purchase. In this example I am purchasing 155 XRP at 0.312727 CAD each XRP.