To unlock the cheat menu you need to go to maddam Hooche for Flying Practice and don't beta the best time but complete the task. Then go straight to the Broom Shed and click on Flying Practice 1 and then beat the best time and get the Wizard card. As soon as you see it press The R button up on top and then press the B button to open up the cheat menu.

NOTE: You can also do this with Oliver Wood, if you choose.