Here is a brief description of the three modes: In Conversation mode, all lines are open and unmuted. In Q-and-A mode, only Organizers can be heard, but non-Organizers can unmute their phone by hitting *6. And in Presentation mode, non-Organizers are muted and have no option to unmute themselves. It's a three-position toggle in this order: Conversation => Q-and-A => Presentation (=> Conversation), and a voice prompt will alert you to the current mode each time you change the configuration.
To call in as organizer, simply press the star key before your access code - i.e. *1234567