Getting an error in MySQL Workbench: Cannot Open Administrator
You are using an old MySQL Workbench version. Please update to the latest version: http://mysqlworkbench.org/2014/04/whats-new-in-mysql-workbench-6-1/.

In older versions of MySQL Workbench we had connections and management instances separated. So it could happen that you have a server instance without a connection assigned. You could edit that instance to re-assign a connection to make it work.

Since version 6.0 this separation is gone. You only have a connection with all the necessary settings for management too.