Expungement would depend entirely on the state the conviction happened in. Different states have different expungement procedures. It'd also depend on if you got convicted of a misdemeanor or felony, too. If it's a federal felony arrest, then you're kinda screwed, but it doesn't quite sound like that. If you were a juvenile when it happened, your state could have "youthful offender" status and just auto expunge convictions. My state, after 10 years, automatically erases the arrest from the public view.
http://en.wikipedia.org/wiki/Expungement If your state is there, that's how it's done in your state.
As far as South Korea, it would depend if you're going through a US Agency and how stringent the US Agency is. Obviously in South Korea, probably the only thing they'd really have knowledge to do is to lookup federal records, so if you got no federal crimes, you should be good, but you'd have to lie I guess. If on the other hand, you're going through a US Agency, with US people doing interviewing, etc, you'll probably have to just explain it to them as you would any other potential employer. They might not even care. Then again, they might, you don't really know.
One thing on Japan, technically Japan will deny entry to anyone with any kind of criminal record from US into Japan. I've heard of people who put down a DUI on the immigration sheet getting handcuffed and denied entry, however, unless your conviction was federal, the likelihood of the Japanese government finding out is pretty slim.
Wish I could be of more help, but yeah.