The final showcase is unlocked once you reach 10,000,000 fans. You will then be prompted to set your waypoint to the final showcase, once you complete the event, the achievement is yours.
The achievement description also says you only need to complete the event, but I came first in my first run of the showcase, so I'm not sure if winning it counts or not, but if it does and you're struggling, just lower the difficulty setting to the easiest difficulty.
If you want to know what you're up against for the final showcase, feel free to click the spoiler.
*EDIT* According to Aki2403, if for some reason you don't want to complete the race, you can quit and still get the achievement.