If you have a VNC config file for the session, you can start the viewer by typing in vncviewer -config < config_file > into a terminal window. If you do not have a config file, read on.
In a terminal window, type in vncviewer This should bring up a dialog prompting you for the hostname. Enter the hostname and hit enter. Alternatively, you could specify the hostname as an argument to the command.
This should bring up a dialog prompting you for a password. Enter the password and hit enter.
This should bring up the remote desktop display.