Well, if it's only links in the Navigation-panel you want that's linking to different forum-categories - then it's very easy to do this.
Log on to your site and go to: Admin Panel->System Admin->Site Links - and make a new link containing:
In the field "Link Name:" you type in eg; "Forum Cat 1" (you can change this to what ever you want)
In the field "Link URL:" you type in this URL "forum/viewforum.php?forum_id=1".
That's it - now you have a link in your navigation-panel that's named "Forum Cat 1" and that's linking to your forum-category 1.
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