How do I install a GameMode?
If you didn't launch the game in at least version 0.28, do that now. Close the game then.
A new folder called "GameModes" should appear in the \Pokemon\ folder.
Drag the folder with the GameMode.dat file into the GameModes folder.
Now start the game up again.
To play this gamemode, simply start a new game and choose the GameMode you want to play in.