Nafplio is about 140 km southwest of Athens, an easy two-hour drive on the motorway via Corinth. KTEL buses run roughly hourly from Athens (Kifissos terminal) to the Nafplio bus station, taking about 2.5 hours. There is no longer a passenger train.
If you are flying in, hire a car at Athens airport — the toll motorway is straightforward, and a car is useful for exploring Mycenae, Tiryns and the rest of the Argolid.