Yes. Just make sure that it is set to act as Markup. If you got it from Mockups To Go, it probably is already set up properly. Read more here: http://help.balsamiq.com/markup.
No, we don’t have a ruler.
We don’t really have guides either. If it was me, I would probably do something with a Horizontal Rule and set it to be treated as markup. You could have it be a dotted line, or something and then indicate what it meant.
Last point: Balsamiq is not meant for visual design. Our goal is to have a rough, quick tool that you can use to pound out ideas quickly so that you are not afraid to throw them away. You can read more about our approach on our site: http://balsamiq.com/products/mockups/#noproto. Hopefully that gives you an idea of what to expect, and whether our tool will be right for you. Thanks!