"남자가 한국에 있습니다."
Translation:The man is in Korea.
I mentioned that here: https://www.duolingo.com/comment/24547746$comment_id=24675944
That would translate like "As for a/the man, he is in Korea." which works, but may not need so much focus on the man. 남자가 한국에 있습니다 is sufficient for "There is a man in Korea", and it was accepted for me, as of 26 January 2019.
I don't know if "There are men in Korea" is accepted, but it should be depending on context (남자 doesn't always have to be 남자들 in order to be plural). Honestly, using 남자는 in that sentence could trigger plural as well. "As for men, ..."
*한국 and yes -에 is the location particle. 있다 means roughly "to be" in the sense of "existing" so this sentence roughly translates to "the man is in Korea". I'd recommend taking a look at the tips & notes for the Basics 2 skill found on the mobile/desktop Duolingo website (not available from the app unfortunately)