The Korea Aikido Federation is headed by Yoon, Dae-Hyun sensei, 4th dan Aikikai, with the headquarters right near the Shinchon line 2 subway station. Yoon sensei's teacher is Yasuo Kobayashi shihan 8th dan Aikikai.
Head on over to www.aikido.co.kr
and you'll find directions on how to get there. There are also more dojos scattered around Seoul, if Shinchon isn't convenient for you - as Peter mentioned, Rupert Atkinson sensei teaches at Pyongchon dojo, which might be more convenient if you live in the southern part of Seoul. You can find directions or contact numbers on the above webpage.
Hope this helps!