Adding field at PO header level

Hi,

I would like to add a field at PO header in 'Customer fields' tab ( right now this tab does not exist as there are no custom fields ). After I read through some documentation I have implemneted enhancement MM06E005 , added field in CI_EKKODM structure. Now the next step is to add field in screen 0110 of program SAPLXM06. How do I add the field here. Do I need access key to modify it?

thanks,

sankar.