Hi,
After you click the keystone tool you should get some over-layed lines on your image, with circular grab handles at either end. Either vertical lines, horizontal lines or both, depending upon the keystone tool chosen.
Using the circles move the lines over the lines you want to be corrected (e.g the 2 sides of your building), and then click the 'apply' button which will be floating over the middle of your image. This should then 'automatically' put the correct values into the keystone tool.
The auto straighten tool is the little tool on the tool bar with a 270 degree anticlockwise circular arrow and a 'plus' symbol in the middle. After clicking on the tool click and drag over your desired horizon line and then let go - should then auto rotate your image.
Hope this helps,
Andy.