Hi,
there are lot of options to select a purchase order in ME22N tcode. You can turn on the document overview and there you can click on Purchase Order then enter the PO number if you dont know the pO number you can search based on all the provided criterias such as created by/Purhcase Org./Plant/vendor etc. You can also click on the Other Purchase Order icon on top of the PO and then enter the PO number if known if not known then search with date or other criterias.
Regards,
Anoop Singh