Leah, the surveys generated with SSI Web are dynamically generated by the SSI Web software when it is uploaded to your server. This prohibits you from accessing the HTML source of these pages which is required within Dreamweaver. If you are testing a survey for layout purposes, and want to play with the CSS, then you can do that in two ways.
First, you can copy a page from the survey by testing out the survey on your local computer, viewing the source in a web browser, copying that source code (along with the included CSS files, if any) into a new Dreamweaver HTML document, then edit the document within Dreamweaver. Then, once you've tweaked the CSS, you can add the modified CSS to your Survey Settings>Headers and Footers>HTML Head Tag section within SSI Web.
Second, and this is my preferred way of modifying CSS on the fly, is to use a browser helper plug-in such as Firebug for Firefox. This will allow you to view and modify CSS code of any webpage on the fly. Of course, any changes you make with this software won't be saved, since you are only viewing a page not editing it. You will then have to reflect those changes in the CSS of your survey by modifying the style or copying the modified CSS to your Survey Settings>Headers and Footers>HTML Head Tag section within SSI Web.
Hope that helps!