The page may help you: Korea Post Office You’ll probably want to click the “English” button in the top right.
Otherwise, write it as given:
101 Dong 103 Huo
7 Dong Seoul
Republic of Korea (South)
The country name is important, otherwise the U.S. post office is likely to return it (unless your local office is used to dealing with mail to Korea). Don’t worry about writing it in Korean; they’ll have no trouble interpreting our alphabet. (General advice: if you do decide to write the address in Korean, be sure that the country name is still in English.)
I tried looking up the proper zip code for your address, but I can’t find a “Sonta” gu. That doesn’t mean the address is wrong; I’ve had trouble with legitimate addresses before. The zip code isn’t necessary, but will speed delivery.
Once it reaches morning in Korea, I expect our Dopers there may be able to help more.