you unlock the mega park after you complete all other scenarioes. it's supposed to be the whole map and i think you start out with all the rides.
the trainers are used for cheating, you can get more money, flatten land instantly and stuff like that. you can download them from a lot of places. one of the best trainers is at
http://www.cse.dmu.ac.uk/~mit00gh/GrottoIE.html. it's called BEAST.