HP 5550 printer on USB does not print

I just removed my Canon S750 from USB (using the wizard) and replaced it by a HP 5550, which is said to work "perfectly" under Linux.
I used the wizard to configure it and every thing seemed ok, but when I click the "test printer" button nothing is output, despite the message saying that a page was sent to the printer.
Note that the HP 5550 works under Windows, and that the Canon worked under Knoppix (notwell but still worked).

Let me do the questions and answers, it may help another beginner like me :
When you configure that printer with the wizard, you have no driver choice : the system chooses automatically foomatic + hpijs.
But .... ijs is not included in the knoppix distribution.
Therefore you have to download it from http://hpinkjet.sourceforge.net, and then follow the installation procedure.