You have to clear the 16th 'Promotional Exam'.
To get to this, on the main menu:
2nd option (Xbox Live)
3rd option (Create a Session)
Top left option, set it to the 3rd option from the top, which is Promotional exams.
Then select the top option. This will search, come to a miniature lobby, and then load the game with a 15 second countdown.
You will get a drop-down menu. Select the 16th option (it may already be selected).
To have this promotional exam unlocked, you must have beaten the previous promotional exam.
In this promotional exam, there are four stages.
1) Get 5 double-line clears in 1 minute 30 seconds.
2) Get 5 Tetris clears in 2 minutes.
3) Erase 8 lines in 2 minutes in 'Eraser'.
4) Clear 40 lines 1 minute 30 seconds.