The voucher is one item that shows you are entitled to use Magical Express, but no particular bus is assigned. This does not matter going to Disney but it is critical that you have a bus time back to the airport. The latter bus time is given on the page or letter delivered to your room. If you don't get the letter at your room on the morning of the day before your departure then immediately go to the front desk to get it (or make any needed changes).
The correct bus time back to the airport is about 3 hours before your flight (domestic) or 4 hours (international). Anything significantly less is a goof that needs correction. You need to be at the DME bus platform 15 minutes before, with everything (carryons, children) all ready to go. Waiting in the line to check baggage (selected airlines) does not count as being punctual.
It is better to wait until you get there before fixing any irregularity with the DME trip going home. This reduces confusion and the chance of something else gettting messed up.