Every practical way to make this trip, cheapest and simplest first.
Take airport bus 6009. At Terminal 1 the stops are 4A and 4B on the 1st floor; at Terminal 2 they are stops 14, 15 and 16 on basement level 1. Get off at Hakyeoul Station, which is the stop SETEC lists for the venue. From Hakyeoul Station Exit 1, cross the car park diagonally and keep going straight for the exhibition halls.
about 90 min · about KRW 17,000 · A single bus most of the way, with space for luggage.
Take the same 6009 airport bus but stay on to the Sinsa Station stop. Change there to city bus 140, 407, 440, 441, 462, 470 or 471 and get off at the aT Center / Yangjae Flower Market stop. If you are already in the city, the easier approach is the Shinbundang Line to Yangjae Citizen's Forest Station and Exit 4, about 50 m from the door.
about 90 min · about KRW 17,000 for the airport bus, plus about KRW 1,500 for the city bus · The route aT Center itself publishes for arrivals from Incheon.
Take a taxi from the rank outside arrivals. For SETEC show the driver 세텍 or the address 서울 강남구 남부순환로 3104; for aT Center show aT센터 or 서울 서초구 강남대로 27. Tolls and the out-of-city surcharge are added to the meter.
about 60 min · about KRW 60,000, plus tolls · Evening arrivals, or when the next 6009 is a long wait away.
Bus 6009 runs every 13 to 45 minutes, so check the departure board before you commit — the first bus leaves Terminal 1 at about 05:40 and the last at about 23:12.