problem when locking the screen

Hi there,
I'm quite new to this mailing list and I have a little problem when I want to 
lock the screen : a dialog box popup which tells me "Will not lock the 
screen, as unlocking would be impossible : No appropriate greeter plugin 
configured."

But I don't find a plugin configuration for this or something else :/
Is anybody else has this issue too?
-- 
Loïc Guitaut <[email protected]>
Jabber ID : [email protected]