1. Draw a curve in the shape you want the bend to be
2. With the shape you want to bend selected - make sure you are in object selection mode - select the bend tool and click on the curve.
You need to have sufficient geometry to make a good bend and it is best used in an orthographic view.
There is another bend tool next to that one which places a cage around the object to be bent - manipulate the cage with the mouse pointer to bend.