In the Virtual Classroom, there is a drop-down menu available in the whiteboard area.
You need to use that option to use content in the class.
Please follow the step-wise process mentioned below:
1. Select New option in the menu and click on Content to access your content library.
2. Choose a file from the Library
3. See preview and share (remember to click on the Share icon to share. Until you share, the attendees won't be able to see the file)