When I used to use the del.icio.us sidebar, you could press Ctrl + B and it would appear. Does the diigo sidebar have a similar shortcut key to make it appear and disappear?
The keyboard shortcut for opening the diigolet sidebar appears to be disabled by default, so you if you open about:config and search for "extensions.diigotb.keys.sidebar"
set enabled to true; extensions.diigotb.keys.sidebar.enabled = true
then control-alt-s can be used to open and close the diigolet sidebar
extensions.diigotb.keys.sidebar.keys = control+alt+s