The right client

The server runs MapleStory v271.5. If the game says it's outdated, or the launcher says the MapleStory.exe size is wrong, your client is a different build.

Get build 26441414 through Steam

  1. Rename Steam's old download folder so the new one doesn't mix with it:
    C:\Program Files (x86)\Steam\steamapps\content\app_216150\depot_216151 → depot_216151_old
  2. Open the Steam console: press Win+R and run steam://open/console.
  3. Type:
    download_depot 216150 216151 9164568599306248228
  4. When it says "Depot download complete", move that depot_216151 folder somewhere of your own.
  5. MapleStory.exe there should be 164,175,328 bytes. Pick it in the launcher.

Then

Start the game from the MapleStory launcher, not from Steam: the launcher signs you in and connects the client to this server. Still stuck? See the graphics help.