I could do it thanks to the info here:
10.6: Get an HP print server working with Snow Leopard - Mac OS X Hints
And no thanks to David Pogue.
I did a cold reset on my Jet-Direct 300x.
Then I changed the ethernet address of my mac in "network", as described, and entered "192.0.0.10" (No idea how that works, when my router has the address 192.0.0.192).
I just wanted to go to the terminal to type "telnet 192.0.0.192" (as described at the above address) when my 6MP suddenly started printing the queue. Without me having been through the whole process.
(I have the impression that the key thing was to switch the Network connection from DHCP to manually, and the number I entered didn't matter that much).
OK, I did it. I don't know how a computer IP of 192.0.0.10 can connect to a print server, whose address is "192.0.0.192", but this is why I'm a writer and photographer and not a computer man.
If somebody can explain that last thing to me, how to different addresses suddenly connect, I'd be glad to know what I just did.