Here:

"An important thing to note is that the Mistlands biome will only generate in areas you have not yet discovered. Therefore, if you have explored a lot of your world, you might be better off starting a new one in order to actually be able to travel to the Mistlands. As per usual, mods will also most likely cause the game not to launch, as they are only compatible with the latest Live version of the game. If you have mods, you will either need to remove them or wait for the mod to be updated before you can play."