I've never tried to edit an existing site with RapidWeaver, but don't think that it is designed for that. You are not able to directly edit HTML with RapidWeaver, but it generates it's own HTML for new pages. Most of the time I find the need to be editing in CSS, which RW is able to do on a limited basis. Most RW power users use CSSEdit for that, which is another rather inexpensive piece of software. You will find that RW is very capable, but limited in flexibility without some additional plugins (again, fairly inexpensive). I consider Blocks, by Your Head Software, to be indespensible. There are many free or very cheap pieces of software designed to extend the flexibility and capability of RW, and the whole ecosystem is much easier to use than Dreamweaver.
There is a very helpful forum for RapidWeaver on the Realmac site (http://www.realmacsoftware.com/forums/index.php/forums/
). Search for tutorials there, and you will find a number of helpful resources. I built my site with a custom theme I developed in RapidWeaver.