Enabling HTML
First enable it generally for a certain user group:
Admin Control Panel > Users and Groups > Manage User Groups > Edit Group > Can use [doHTML] tag? > Yes
Now you can enable it for specific areas, such as:
1. In the forums:
Admin Control Panel > Forum Control > Manage Forums > Settings > Allow use of the [doHTML] tag? > Yes
2. In Signatures:
Admin Control Panel > System Settings > User Profiles > Allow HTML in signatures? > Yes
3. In PMs:
Admin Control Panel > System Settings > PM set-up > Allow HTML in messages? > yes