Taxi (from both airports) is always a feasible solution and but it's not
recommended at all unless you have no other options.
For detailed info on how to come to SIM from the airports,
please read this guide page (PDF).
Here is also a PDF file saying "you want to go to 1) SIM, 2) Yonsei univ. main gate, 3) Somerset Palace Seoul" in Korean. You may print this out and carry it just in case you need to ask some local people for help.
VENUE
CIAA 2016 will be helded at Cypress Hall (2F)
in Sangnam Institute of Management (SIM).
Building No. 414 on the following campus map (click the map for bigger
size). Note that M at
the bottom in the map indicates
the main gate of Yonsei University.