1. Install the Fireboard component
Extensions->Install/Uninstall->Install from URL:
http://joomlacode.org/gf/download/frsrelease/4719/11072/component_Fireboard_Forum_1.0.1.beta.zip

2. Go to the Fireboard component and install the database:
Components->Fireboard->Click on "Clean install"

If, when you go to the "Forum administration tab", you get an SQL error, see this post:
http://www.bestofjoomla.com/component/option,com_fireboard/Itemid,38/func,view/catid,65/id,12610/#12610

3. Integrate with CB like so:
Fireboard->Configuration->Avatar Integration: Set to "CB"
Fireboard->Configuration->Profile Settings: Set to "CB"
Fireboard->Configuration->Community Builder->Community Builder user profiles: Set to "Yes"
Fireboard->Configuration->Community Builder->Create Community Builder forum profile: Click!

So:
4. Install the Fireboard for CB plugin.It's available here:
http://www.bestofjoomla.com/component/option,com_sobi2/sobi2Task,sobi2Details/catid,5/sobi2Id,70/Itemid,41/
Components->Fireboard->Plugin Management->Install New Plugin
Components->Fireboard->Plugin Management-> CB Fireboard Plugin : Publish
Components->Fireboard->Tab Management->Forum : Publish (NB: make sure you publish the Fireboard one, not the SimpleBoard one!)

IF you get weird tab names like "_UE_FB_TABTITLE" under "My profile", then you need to edit the CB language file:

If you want it in simple default english simply cut and paste this into your CB language file. Normally:
"components/com_comprofiler/plugin/language/default_language/default_language.php"

//Fireboard Forum Tab Language Settings
DEFINE('_UE_FB_TABTITLE','Forum Settings');
DEFINE('_UE_FB_TABDESC','Change your forum settings');
DEFINE('_UE_FB_VIEWTYPE_TITLE','Select Viewtype:');
DEFINE('_UE_FB_VIEWTYPE_FLAT','Flat');
DEFINE('_UE_FB_VIEWTYPE_THREADED','Threaded');
DEFINE('_UE_FB_ORDERING_TITLE','Post Order:');
DEFINE('_UE_FB_ORDERING_OLDEST','Oldest First');
DEFINE('_UE_FB_ORDERING_LATEST','Newest First');
DEFINE('_UE_FB_SIGNATURE','Signature:');
 

5. Add a menu item!

Menu->Menu Manager : Select the menu in which you want the "forum" link to appear.
then New->Select Menu Item Type->Fireboard

6. And finally, add some forum categories

Components->Fireboard->Forum Administration->New

Note that first you need "categories" and then "forums" - read the instructions under "Parent"