
There are two ways to plot people. The first is to pick a root person, set a number of generations and then click Run, then wait. The other way is to click a pin for a person whose parents are not yet plotted and then click the big green "Expand Parents" button. Once a person's parents have been plotted, the green button will disappear.
If you have already plotted 7 generations, you can click any #7 pin and expand beyond 7 generations (up to a max of 20 at the time of this writing). If you want to just plot a single line, start with a 1 generation run and then expand out 'til your heart's content.