The AP Div or, more correctly, the absolute positioned element was grossly misused in earlier versions of Dreamweaver. It was used as a visual layout tool with disastrous results. This is the reason why you will not find it in recent versions of Dreamweaver.
For more information on the position property, have a look at CSS Layout - The position Property
BenPleysier is right to say that absolutely positioned elements were grossly misused. After studying how the position property works, if you decide you need to use absolute positioning, you can still do so in Dreamweaver. Just give the element an ID, and create a style rule for it in the CSS Designer.