so if i guess right, firmware was already installed and you just added display without doing anything else.
so that cannot work. do you already have arduino ide installed on your computer ?
it´s not just "plug and play" , the controller you have is not included in arduino by default.
so you need the support for your melzi board installed also.
anyway , ask the dealer you bought the printer about firmware files, just to see what´s actually installed.
I´m not familiar with the melzi board , so maybe you´ll find some support respectively how to add the board in arduino forum