You edit the signature within your CB profile, not your Kunena profile. You need to enable all integrations for Community Builder within Kunena. This works and is tested working (we're using it here at Joomlapolis). Once Kunena is configured navigate to your CB profile then edit profile and you should see the Forum tab. If not displayed please check that CB > Plugin Management > "Forum integration" is configured to use Kunena. Then check that the CB > Tab Management > "Forum" tab is present, public, published. If all ok please then check that the forum fields are present and enabled in Field Management.