Dreamweaver does not have any automated way to create a forum.
Unless you are very skilled in server side scripting languages like php and getting them to talk to databases like mySQL, your best route is likely to speak to your hosting company and see what/if they have available in a pre-made forum package.
If they don't have anything available, you can find them online for a fee.
There are a number of free pre-packaged forums available. All depend upon some kind of server-side scripting language (ie, PHP, JSP, ColdFusion, ASP, etc.) Much better than trying to build one from scratch.
DW doesn't have a magic button to create interactive forums. To do this from scratch, you need advanced coding skills & a thorough knowledge of server security. Which server-side programming languages and databases do you currently work with?