To monitor the system controller of a sf6900 you need to use a platform agent. A platform agent will run on a box of your choice in the environment and proxy the system controller. Manily the SUNW is SUNWesspa. But during the es-inst simply say yes to monitoring the Sun Fire Midrange Platform.
Next follow the SunMC supplement guide for this platform on docs.sun.com. It walks you through configuring the system controller to talk to the SunMC server. It also covers how to setup a platform agent.