How do I change the ink cartridge on Epson Stylus D120?

One of the 2 black cartridges needs changing - the middle button light flashing. Pressed middle button which should bring cartrdge holder to the middle so cartridge can be changed - nothing happened. Repeated several times. Switched off machine, than on again - now middle button not flashing, still no action when pressed. Tried switch off and on again, same thing. How do I get the cartridge holder to the middle so I can change cartridge please?