First you have to save up around 5000 MVP Points. Save your profile. Then buy the Secret Players that you want to get and put them on a team. Then save the rosters. Now load your profile again and you will still have your original amount of MVP points and the player will still be on the team.
From the "My MVP" menu, Create a player named "Dan Carter". Upon entry of the name there will be a message indicating that the code was successful.
Create a player called Katie Roy and you will have unlocked all rewards.
In Owner Mode, purchase all the available Ballpark Upgrades (Make sure each upgrade has reached its maximum level). After purchasing your final upgrade, an extra option will appear in the Buy Assets menu. This new Option will be called Secret Stuff.
Unlock all 25 Legends Team players to unlock the Legends Team. Unlock all 25 Heroes Team players to unlock the Heroes team.