Once logged in, you'll be presented with "Choose your audio & video options". Select PC Audio/Video and you'll be able to set your Microphone, Speakers and Video preferences.
Once you've confirmed your preferred settings, click Connect to join the audio & video portion of your meeting.
If you prefer to join the audio portion to begin with, toggle the "Use video in this meeting button.
You'll be able to enable video at a later point from your Online Meeting controller.