I looked at the RoundUp website and they moved to 서울시 마포구 서교동 455-15 2층
Which is somewhere by Mangwon Station. Get out of Exit 1... and who the heck knows from there.
Try inserting that address into map.naver.com and it'll help you out.
Hongdae has a Kasina Premium store and Ronin is located right across the street. Nike NSW is also there.
You can check out Premium, Pinnacle, Stussy, Liful, and Worksout in Apgujeong. They are all fairly close to each other so it's a pretty good stop.