How to continue drawing on a line that you left unfinished?

Say I've drawn a few curves on a layer with the bézier tool, and I want to go back to some that I did earlier and add to them. I'm trying to do this and it won't let me; it wants to start a new line.

I'm really trying to figure out what I'm doing wrong, but I keep doing the same thing that isn't working. Please enlighten.

Thanks,
Ellen