These are the steps required to create HTML5 sessions/events :

1 - After logging in - In the Account Center - Click the New Meeting button.

2 - Give you meeting a name and start time as normal. 

3 - Click the Advanced Options button ( note this may not be available on all subscription plans ). 

At the button of this form - you will see the two room type options 'Flash' or 'HTML5' - you want to use the HTML5 radio button to create the HTML5 session.