How do I draw an arrow in Acorn?
Acorn doesn't really have native support for complex shapes / arrows like Skitch does - however, you can make an arrow using the Bezier pen tool.  You'd select it from the vector tools palette, and click around until you got your arrow.  It'll be super annoying to rotate or resize however, and it probably won't look as good.

Adding something like this has been on the todo list for a long long time, so hopefully it'll happen in a future release.

-gus