Do you need a dictionary application or system plug-in for Apple Dictionary application?
You can use hunspell dictionary file or ko-aff-dic-file with dictionary app.
Although they are meant to used as a Korean spell checker fuction they works as a dictinary as well.
You can easily find other DIC files like Korean-English, English-Korean and more on the web with Google God.

Read following links to how to extend Apple Dictionary's functions.

http://baram.or.kr
http://blog.naver.com/sixt06/70068316248
http://code.google.com/p/spellcheck-ko/downloads/list