Hi Alexander,
I think global object services (GOS) are not activated in DocuLink customizing.
You will find this in transaction J6NP, displaying a version of a project and choosing menu Extras->GOS.
You will need to have a project/view for SAP object type BUS2010 (request for quotation) for activating it in transaction ME43.
ME23N uses SAP object type BUS2012; so you will probably cannot use the existing view of the purchase orders.
Regards,
Benedikt