Minecraft – How to fly in Creative Mode

As of Minecraft 1.3, singleplayer and multiplayer have been merged, so you can switch your gamemode with the /gamemode command:

• /gamemode survival
• /gamemode creative
• /gamemode adventure

You can abbreviate the modes. For example, typing /gamemode c for Creative works as well.

You can only do this if you haven't disabled cheats during world generation, though.

If you turned cheats off and still want to do it for whatever reason: The game mode is stored in the level.dat file, which can be edited using an NBT editor, such as NBTExplorer. The game mode is stored in Data/GameType.

You could try using the /gamemode <name> 1 command to set your users to be in creative mode. You'll need to do it for each user. Or you could do it like so: /gamemode @a 1 if you want all players to be in Creative mode.