LibreOffice 3.5.72 - non printing characters

LibreOffice 3.5.72 - non printing characters

I am trying to visualize non printing characters on LibreOffice 3.5.7.2
Build ID: 350m1(Build:2) in Ubuntu 12.04
According to the help page and to what I have found on line, I should
either press Ctrl + F10 or go to Tools - Options - LibreOffice
Writer/LibreOffice Writer/Web - Formatting Aids
As an other alternative, I could modify the standard bar (adding the
desired tool).
The problem is that none of these is available! No relevant icon in the
standard bar, no LibreOffice Writer in the options, no function linked to
Ctrl + F10!
Am I doing it wrong? Do I need to update Libre Office?