There are a number of valid approaches and possible protocols, which you can use to generate sets of styles for a Media Query in Dreamweaver CS5.5 and CS5. Before walking through one that works well, let's survey what it is we are trying to accomplish.
Essentially, we need three sets of style sheets: one for phones, one for tablets, and one for full-sized monitors. Both because full-sized monitors are still, at this point, a "standard" in terms of how the web content is viewed, and because the phone and tablet styles will likely have less complex styles, it makes sense to start by creating a style sheet for a full-sized page.
In other words, the first step is to create a basic CSS Style sheet that works well with the page when it is opened in a full-sized desktop or laptop monitor. Then, variations on that page can be generated with simplified layouts that work on phones and tablets.