Up on the top...see the down-arrow by the pencil? What do you see if you click the down-arrow? (I've never used paint.net)
Right now, that pencil is 1 pixel in size - that's what it's telling you at the bottom (sorry if i'm insulting your intelligence - it's not intentional)
When I get back from the chiro, I'll look up paint.net and see what I can learn
