There are 2 ways to attach a stylesheet to an html page ... 1) Link, and 2) Import
This will link the style sheet to the page but ... pre version 5 browsers which do not recognise CSS properly may have problems displaying the html coherently. The linking code will appear in the <head> tag and look like this ...
<link rel="stylesheet" href="styles.css" type="text/css">
This will link the style sheet but hide it from any non css compatible (pre version 5) browsers which will simply ignore it. Your end-user will then see simple but logically formatted html. The linking code will appear in the <head> tag and look like this ...
<style type ="text/css">
Open each HTML page you want to attach the CSS file to and use the Attach Style Sheet button at the bottom of the CSS palette to attach the CSS file to each page.
Dreamweaver will attach (link) the CSS file to the html page currently selected/in focus.