I finally made the videoconference (DimDim) working. Here are the basic steps:
1. Register an account at dimdim website to get a username and password.
2. Login as Admin. Go to Configuration > Chat and Video Conference tab > enter the username and password to the DimDim Setting.
3. Login as Instructor. Go to Collaboration Area > VideoConference. Create the VideoConference room. See screenshot below.
4. Click Enter and Start Meeting link. This pop up a new window. You must start the meeting, else student cannot join the VC.

5. Below is the VideoConference interface where both instructor and student have joined. Right side is the Chat Room. Middle portion can be
Computer Screen (like PC Anywhere, intructor's screen will be displayed on Student Machine) (i couldn't get in because DimDim said the network was congusted), White Board (Both Instructor and Students can draw or type something on the whiteboard). Left Bottom portion is the list of Participants and Camera (i don't have camera, so blank).

6. Simultaneously, students can login, choose the course, go to Collaboration Area > VideoConference. Click Enter to join the VC room.
