Ben Spies isn't available to use in single player until he's unlocked at level 56. However, he can be used in multiplayer at anytime.
Firstly, from the main menu, choose championship. Then choose Moto GP from the next menu. Now select official riders. Highlight Ben Spies from the next menu and then exit out until you are back to the main menu.
Now you need to enter multiplayer mode and set up your game. It's probably best to get someone to boost with you at this point as quite often the lobbies are empty and you need a certain bit of help to get the correct options required.
When there is enough players in the lobby (you and one other minimum) you get the track selection screen. Choose Circuit Of The Americas aka Austin. If you are doing this with random opponents you have to hope you win the track vote. Next will be the weather options with clear being the easiest and fastest option.
Now it's the rider select screen. Ben Spies should be already selected for you.
Now win the race and the trophy should be yours.