The Main Event occurs on Talent Shows each Saturday.
To win the Main Event, you need to average a gold score over the entire show. The Main Event is a little harder than most Talent Show games, but not by much.
In order to unlock this achievement, just sign in to the game two Saturdays in a row and win the Talent Show. You are not required to play any talent shows in between, but you can if you need to.
You can either do this in real time, or, you can use the offline system clock settings to change the dates on your xbox. To do this, unhook your internet connection, go to System Settings, Console Settings, and then Date and TIme. Change the date to a saturday. From here, you win the Talent Show and repeat the process, choosing the very next Saturday.
Also note that Main Event shows give you about twice as many Trainer points for winning, so it will help you level up a little faster as well.