Go to Control Panel > Forum Permissions

Within the following two sections, enter a 1 for each of your user groups that you'd like to allow that feature for:
"Can post using HTML"
"Can post using Markup/UBBCode"

In all normal situations, I would advise against allowing HTML posts by any user that is not an administrator. The need for posting pure HTML on a BBCode-enabled forum is very rare, and it enables abuse of security related isues.