I just arrived on Seoul and brought some packets of dried mangoes for my hosts. I forgot to wrap them however. I've read that Koreans are particular about their gifts being wrapped but these gifts being just snacks, I was wondering if I really have to wrap them. I'd hate to offend them.
You're not going to "offend" anybody by giving them an unwrapped gift, but careful packaging will definitely increase the gift's perceived value and the brownie points you get for giving it, and yes, this extends to "just" snacks.
Quick primer: http://www.korea4expats.com/article-gift-giving.html
If you're staying in a hotel, reception can probably wrap them for you or at least tell you where to get them wrapped.