Getting to Rabaul requires flying, and the logistics are worth planning carefully given PNG's transport complexity.
By air: Tokua Airport (RAB) is about 45 minutes from central Rabaul by road (the road passes through Kokopo). PNG Air and Air Niugini both operate flights between Rabaul and Port Moresby, usually with one or two per day. Connection times through Port Moresby from international destinations can be tight — PNG Air and Air Niugini share the same terminal in Port Moresby and connections are technically possible, but PNG domestic schedules change frequently and building in a buffer day in Port Moresby is recommended for international travellers.
There is no passenger ferry service to Rabaul from the PNG mainland. Cargo vessels occasionally take passengers but schedules are unreliable.
From the airport: PMV (public motor vehicle, essentially shared transport) runs between the airport and Rabaul/Kokopo. Guesthouses will often arrange airport pickup if contacted in advance.