When you'll come from Turku, Helsinki, Tampere or Hämeenlinna go first to Kaarina and then from there to Parainen/Korpo direction, onto the Arghipelago Ring road. Just follow that road to Korppoo direction and after city center of Parainen you'll end up to a ferry (15 km), Parainen - Nagu ferry. During summer times it runs like every 15 minutes, other times littel rarely. The trip on ferry lasts only 10 minutes, here is the link to ferry timetable.
After ferry it takes 15 minutes driving (by car) to Nagu center, and over there right after when you past Nagu church, on first intersection, make a right turn (road name Kyrkvallen/ Kirkkovalli) and you'll see Majatalo Martta on the left at top of the hill.
You can came to Nagu from other direction too, from Korpo, via Arghipelago Ring road too, you'll end up to Korpo - Nagu ferry. After ferry it takes 15 minutes driving (by car) to Nagu center, and over there the intersection just before Nagu church, make a left turn (road name Kyrkvallen/ Kirkkovalli) and you'll see Majatalo Martta on the left at top of the hill.
Also during summer time, you'll may get to Nagu from Naantali direction, at first you have to go Rymättylä and from there all the way to Hanka harbour. From Hanka you have to take a ferry that's named M/s Östern to Nagu. The ferry M/s Östern's trip takes an hour and on the way it stops at Seili Island. This is very interest Island itself to visit because of it's old history as hospital for spitals and also it's nature is very rare. The ferry M/s Östern ends up to Nagu's guest harbour, and from there is like a 300 meter (less than 1/4 mile) trip to Martta's Inn.