You have to clear the 15th '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 15th 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) Clear 5 rounds of 'Target' in 2 minutes 40 seconds.
2) Clear 35 lines of 'High Speed 2' in 2 minutes
3) Erase 6 lines in 2 minutes in 'Eraser'.
4) Clear 5 rounds of 'Target' in 3 minutes.