Continuing paths on a shape with the Pen?

[media kat]

After drawing a shape with the Pen tool I would like to modify the shape by continuing to add paths to some of the existing anchor points, however, when I place the cursor next to an anchor point it turns into the minus sign and when I click it deletes that anchor point. How do I add to existing anchors on a shape?

Thanks

[Ton Frederiks]

Maybe Preferences > Disable Auto Add/Delete and zooming in helps.

[Met1]

Adding to Ton's insight, you can also use the pencil tool to do small refinements, just start drawing right over the top of your existing line to make modifications. I find it works better in small sections rather than trying to revise a whole path in one go...