I write down here the complete procedure, to help anyone will encounter this problem.

- First of all, make sure you have the IP address of the Printer you want to set up;

then open a browser and type in the URL bar:

localhost:631

Now you will gain access at CUPS Printer Server of your Ubuntu system.

To add a new printer, click on
ADMINISTRATION tab

Add printer

Scroll down the page and under "Other network printers" select

AppSocket/HP Jet Direct

and press Continue

Now type the IP address of your printer, like this:

socket://xxx.xxx.xxx.xxx

and click on Continue

Now just type the name of the Printer as you want to see it on your sistem (e.g. : Konica_C280)
and click on Continue

Now you have to choose the driver, under "Make" (I guess it's "make" in english, I see it in italian) search the list for GENERIC

and then click on Continue

And now in this other list search for Generic PostScript Level 1 Printer Foomatic/Postscript1 (recommended) , which is the one I successfully used.
Then click on ADD PRINTER.

Now that you have added the printer, click on the "Interrogate printer for default options" button on the upper left and you should now have a totally functional Konica Minolta C280 printer set up on your system.