If you want to take buses from Seoul to Yongin, then you need to go to the Seoul Express Bus Terminal. The timetable, price and other information about the bus services are available at the following website. The buses are generally comfortable, try to pay more and take the better class buses.
Since Yongin Everland is located on a hilly area, there are ski-lifts at various locations where you can take up or down the slope instead of having to walk. However, the queue can be rather long especially near the time when the park is going to close.
Honestly speaking, getting here is a bit of pain as Yongin is some distance away from Seoul. But it's not all that difficult. All you need to do is to proceed to Dong Seoul Bus Terminal (Gangbyeon Station (Seoul Subway Line No.2), Exit 4 ), purchase a bus ticket from the many counters there and take an express bus bound for Everland (1 hour ride). For your return trip, there will be lots of buses there that make trips back to the nearest train station (Suwon) that will take you back to Seoul. Directions and everything else are written in English so there's no need to worry.
Official Everland Address.
310 Jeondae-ri, Pogok-myeon, Yongin-si, Gyeonggi-do