Since you've posted this in the MOC forum, you have a couple of options within MOC:
1) In you contact list, right click the person you want to do a video call with and select "Start a Video Call"
or
2) Start a session with a contact and then click the camera on the menu bar.
Note: Make sure that you've set up the camera and that you've set up MOC to use the camera (Tools - Set Up Audio and Video...") first.