Adding a field to a crystal report in sap b1

All I am trying to do is to add the field for credit limited to the standard item invoice in SAP 8.8,

I have added the BP table OCRD and draged the creditline field to the layout, but when you view all the fields are empty.

I obviously need to link somthing - I have excepted the links that are automatically completed by SAP.

Can anyone point me in the right direction.

Thanks

Jo