Unlike websites with which you are familiar, DotNetNuke builds pages dynamically as you need them. Therefore, there aren't pages, per se, that you can work with. As a result, there are not pages that you can manipulate with something like Dreamweaver.
You could edit things like Text/HTML contents with Dreamweaver, but that would probably be overkill.
I'd first suggest that you become familiar with the basic concepts of how a DotNetNuke site is put together. For that, take a look at these tutorials:
As for editing a site, mostly it can be done on the live site. That is particularly true with version 5 of DotNetNuke that includes versioning in the Text/HTML Module. You can edit modules, but keep the content off of the live site until you are happy with it. You can also create and develop new pages that you keep invisible, via security roles, until they are ready to be made live.
For large and complex alterations, you have the option of bringing a copy of the site to another server, making the changes, and then uploading the altered site to your live server. For something like that, see:
As for editing, I generally use the tools built into DotNetNuke. If I need to work with a big chunk of HTML, I'll often copy and paste it into an html editor to do that offline. I use Visual Web Developer, but Dreamweaver or any other HTML editor would be fine. Sometimes I just use Notepad++.