Here's how it works:
1. WinKey + R
2. Type in regedit, press enter.
3. If you are using Vista or newer, make sure to OK / Yes the UAC dialog.
4. Navigate to HKEY_CURRENT_USER\Software\Nexon\Mabinogi in regedit
5. Find Width and Height, and change them to the resolution you desire.
Do this only if you wish to keep fullscreen, and Mabinogi doesn't support your resolution.