Currently there is NO direct bus connection from Rijeka to Plitvice lakes, what you have to do is to travel from Rijeka to Karlovac (Bus connection is listed below) and then change to a bus direction Plitvice from there on, you can see bus departure from Karlovac to Plitvice here
Alternative to taking the bus, you can consider renting a car and drive to the national park on your own, you can see car rental offers from Rijeka and Rijeka airport here.