Apr 10, 2011 at 8:38 AM
You need 10 posts and to have been registered for 7 days to meet the requirements. Then, go to User CP and go to Edit Options.
Most people probably don't realise this, but this forum has had two editors for a number of years. One is the xenForo default editor, and the other is a custom editor I made years back I called BBCEd.
All the settings for which editor you use was lost during the upgrade. You can find the setting under Account Settings > Preferences > Editor.