I am assuming you are asking about positioning the div and not setting the alignment of its contents. In order to either float or absolutely position the div, you will have to use CSS.
Let's imagine you had a div with the id of main like so:
If you open the CSS Designer panel, you will need to add a new selector to your stylesheet for this div (if you haven't already done so, you'll need to create a new stylesheet or add the styles in page via the "sources" pane). Now with the div selected, click to create a new selector in the selectors pane and it should add a selector called "#main". At this point, you can view the properties pane and choose the layout option which will give you the avialable positioning properties such as float, top, left and so on.