To get to Namiseom from Seoul, you'll have to reserve an ITX tickets and get off at Gapyeong station, then take a cab or bus to the ferry station, and then finally make your way over to Namiseom.
To get to Petite France, once you head back to the mainland from Namiseom, you can either take a bus or taxi. There isn't much to do there, but if you're a huge kdrama fan and you want cute photos, then it might be worth stopping by.
P.S. I highly recommend that you rent out a bike when you're at Namiseom. I promise you won't regret it and it'll make the trip 10 times better.