You can't select the difficulty in this campaign menu, however there exists a method to change it. If you doubt my words, please press F11 to check the timer/difficulty. The game actually keeps and uses the difficulty of the last scenario you played in the official campaign section, so what you need to do is simply play a scenario in the campaign with the difficulty you desired and finish/quit the level, then you can play the AoW missions with the difficulty you wanted. For example: You want to play "Destroying a Castle" on Moderate, then you need to play a scenario in the official campaign (e.g. "Marching and Fighting") on Moderate first so the game will remember that data. Now when you play "Destroying a Castle" it will always put you on Moderate. If you want Hard or Standard, just do the same.