To have attendees muted by default when they join, simply go to the options tab while creating the meeting and select more options.

Select "Roles", and disable the Allow mic and Allow camera for attendees options. Then hit apply and finish creating the meeting.
Please see the example setup shown below.

How to Edit an Existing Meeting:
To edit the settings on a meeting you have already made, click edit the meeting from your calendar, and ctrl + left click the meeting options.

In the web page that pops up, click Roles and then edit the settings under Participation and hit Save.

For instructions on how to create a meeting, please navigate to canton.teamdynamix.com/TDClient/2726/Portal/KB/ArticleDet?ID=135020.