It pretty much depends on what version (and to some degree what skin) you're using, and how much you want the link to the forums to integrate into the sidebar.

You can make up a blog and call it "forums" and give it the URL for the forums index.php file so that it integrates with the bloglist.

You can make a post in your linkblog and link to your forums so that it gets added to your linkblog list.

You can, depending on your version, use good old fashioned html to add it to your sidebar as a standalone link.

You can, depending on your version, use the Free HTML widget to use good old fashioned html to add it to your sidebar as a standalone link.

I reckon there are other ways too. These are the easy ones that pop in my head at this moment without more info (like a link yah?) about your specific installation and desires.

Thank you sooooo much for your help !
I'm using the latest version of b2evolution (2.4.1) and the skin I'm using is based on evocamp skin.
I'll will try to implement one of your solutions and see if it works :)
But I think the Free HTML widget doesn't work in the navigation bar does it ?

I tried using the Free HTML in the "Menu" section and it worked fine. I'm assuming by "Nav bar" you are referring to the area where the Home and Contact links are.

http://glassguides.com/blogs/ (Architects' Guide to Glass & Metal link)

I had to add a couple of   in front of it to space it properly.

BTW - that picture on the blog isn't me. That is one of the magazine editors that works for the company. I'm the behind the scenes "web guy."

The Free HTML widget does the job just fine, thanks for your help guys !