tried the MBP with the most important print jobs with the printer, and it is not showing loaded paper and taking hours to process print. Maybe there is something wrong with the printer after all, not just cables and driver. Or, it is lousy drivers, I do not know. Must contact support next week.
I managed to print a good print and it started to print immediately, but someone else´s MBP does not work as described above.
My mba did not manage to print either. Just a week ago it worked fine with some A3 sheets with black and white I had previously printed on Epson 4900.
Next question: I looked for hp printer driver downloads from apple, and there is actually one: http://support.apple.com/kb/DL339
I have not dared to download that. Found on http://support.apple.com/downloads/#printer%20drivers
(when I go to system choices and printers, to the list of printers, and select printer and - , the z2100 should re-install from apple updates (If I plug in the usb cable) but it did not do that. I tried the other MPB, and the printer driver did not come via the app store thingy, there was a small pop-up asking if printer driver should install. Maybe it just installed upon plugging in the usb cable, maybe because I have selected install updates automatically?? Maybe. Must try unchecking them next time.