I can't remember exactly how to get there, but from memory if you can read the subway map it isn't very hard! There is a station right next to the complex, follow the crowds to reach the mall.
Was a little disappointed when I was there a few years ago - surprisingly prices were more or less on par with NZ minus the GST. However the USD is significantly stronger now than back then so there may be more bargains to be had! Also definitely the place to be if you are looking for more esoteric gadgets.
Are you staying in Seoul? One thing to note is that Seoul itself is a long, long way from Incheon airport. There is a train service now (was being built when I was there) - allow plenty of travelling time. And keep in mind the trains can be manic around the beginning and end of the business day.
Oh and if you have left over time I'd recommend a visit to Seoul tower - best way to take in a great view of the city and countryside. Again can be reached by train - little bit of a hike from the nearest station from memory. Other sights worth seeing that are easy to get to by train are the palaces and the war museum. The museum is massive - I ran out of time there just looking around at the exhibits outside, never got a chance to go inside!