You are doing nothing wrong. The trial version does not support MPEG. The full-paid version has no such limitations, and also comes with a 30-day, money-back guarantee, so if you do not like it, you can return it for a refund.
The limitations of the trial version are HERE.
Good luck, and welcome to the forum.
Oh dear. Ok thats good to know at least. Although now i have to travel back to london to get it done which is annoying!
Hey, travel to London is not all THAT bad...
Sorry for the bad news, and do remember that with the full-paid version, one has 30-days to put EVERYTHING to a test - no limitations, and Adobe will refund that $ (or £), so long as the product has not been used over 30-days.