Instructions
Speak with McGonagal to begin a cutscene, where she will inform you that you do not have permission to go to Hogsmeade.
Exit the room and you will notice a blinking red dot on the left side of your mini map, indicating the presence of Filch.
Tap the Invisibility Cloak icon on the bottom right of the screen with the DS stylus. You cannot cast spells or run quickly while the cloak is activate.
Head left, keeping as much distance between you and Filch as possible. Make your way through the front door, past Filch, and Fred and George will appear to give you the Marauder's Map.