Adding any link to the navbar isn't too bad of a process.
Open
/templates/{your template}/header.tpl :
Code:
After the {/if} statement and before the </ul> Add:
Code:
<li><a href="/forum/">Forum</a></li>
Of course you'd change the link from /forum/ to wherever you installed your forum.
I did this on my ProductJump site for the "Contact" link.
If you put it within the {if $user_authenticated eq true} {else} {/if} in header.tpl, it would either show up if someone is logged in, or not logged in.
Granted, this is a direct and easy way to add a link, and certainly not the "official" method.