It is a bug (glitch), and already reported to MySql team. I had the same problem, by the time it get fixed; Please use it by "restore down" window mode.
When defining a user the "Schema Privileges" tab is non-functional when the Workbench window is in fullscreen mode (which is the default mode on start-up). Only the "Schema" column is shown in the embedded window and the "revoke all privileges", "delete entry", and "add entry" buttons are missing.
The problem goes away when the "restore down" window button is clicked and does NOT re-appear when the Workbench window is returned to fullscreen.