Where do I go to register for the National Health Insurance in South Korea if I am there on an intra-company transfer visa?
There's a pretty good guide here on the NHIC website. It should walk you through the entire process. You should be able to find your specific visa information there. Otherwise, you can call the NHIC hotline. They have an English line number at 02-390-2000.
There is also a good guide at Korea4Expats. The information should be fairly up to date.