The only way to get to Unzen via public transportation is by bus. From Nagasaki, buses depart three times a day from the Ken-ei Bus Terminal (tel. 095/826-6221) across the street from Nagasaki Station (sit on the right side of the bus for the most panoramic views). The ride takes about 1 hour and 40 minutes and costs ¥1,900 one-way. If you have a rail pass, take the train from Nagasaki to Isahaya Station, go straight out of the station and take the stairs on the right-hand side to cross the street, and then walk down the stairs to the Shimatetsu Bus Center (tel. 0957/22-4091), where you can catch one of 10 buses a day to Unzen at a cost of ¥1,300 for the 75-minute trip. Ask for bus and train schedules at the tourist information counter at Nagasaki Station. And by the way, both these buses pass through Obama, which sports flags with "I heart Obama" and a cartoon drawing of the U.S. president.
You can also reach Unzen from Kumamoto. From Kumamoto Port (which you can reach in 25 min. by bus from Kumamoto Station for ¥420), take the Kumamoto Ferry (tel. 096/356-2755; 1 hr.) for ¥680 or the faster 30-minute Ocean Arrow for ¥800 to Shimabara Port, where you can catch a bus bound for Unzen, which takes an additional 40 minutes and costs ¥730 one-way.
Pick up the free leaflet "Nagasaki, Unzen, Goto Islands, Iki and Tsushima" from the Tourist Information Center in Tokyo or Kansai or Narita international airports, or download it from JNTO's website at www.jnto.go.jp by looking under "Browse by Destinations."
In Unzen, the Tourist Information Center (tel. 0957/73-3434; www.unzen.org; daily 9am-5pm), is located in the heart of Unzen Spa next to the Unzen Spa House and across from the police station (btw. the Nishi-Iriguchi and Oyama-no-Johokan bus stops). Not much English is spoken, but it has a good English-language map of Unzen and the surrounding area, a hiking map, and an English-language audio guide for ¥200 per day that provides information on sights and history. Nearby is the park's Mount Unzen Visitor Center (tel. 0957/73-3636; Fri-Wed 9am-6pm, to 5pm Nov-Mar), with displays and information on the entire national park and a knowledgeable park ranger who can provide hiking information.
Buses from Nagasaki make four stops in Unzen Spa, terminating at Unzen's Shimatetsu Bus Center (tel. 0957/73-3366). I've provided the bus-stop name for accommodations so you can alight at the nearest stop. Otherwise, Unzen is so small you can walk from one end to the other in about 20 minutes.