Unfortunatly there are no direct schedule bus between Umag and the National park Plitvice Lakes, the easist way to the there by bus is to join an excursion / day trip from one of the travel agencies in Umag. If you insist on travelling by public bus, you will have to first travel from Umag to Karlovac, or alternativly from Umag to Rijeka, then Rijeka to Karlovac, from Karlovac you then have buses to Plitvice