I can't tell you how to do this through the Dreamweaver interface, but I can tell you how to do it in HTML. To insert the nav frame on each desired page, use the following HTML in the appropriate position on each page:
HTML Code:
<iframe border="0" source="SOURCE.htm" height="HEIGHT" width="WIDTH" />
To answer your second question, yes, a CSS stylesheet may be used on multiple pages at one time. To do so, insert the following HTML into the HEAD section of the HTML document:
HTML Code:
<LINK REL="stylesheet" TYPE="text/css" HREF="STYLESHEET.htm" />
Please note I have made every possible attempt to make the code given here as W3C compatible as possible, however, I do make mistakes (and I don't normally use HTML that is W3C standard, but when giving HTML advice I try to

).