As the others have said, you do not have to contact Magical Express 24 hours before your return trip. I was booking Magical Express just today, for our family trip in February. I asked the CM the same question you are asking.
She said you should have received your return trip bus vouchers in the mail, with your ME package. These are all you need to board the bus. She also said the resort will hang a reminder of your departure time on your room door, the day before you check out.
Have a good trip.