The first 4 courses are free but when you must clear the previous stage to unlock the other 6 courses.

Course	How to unlock
Course 5	Clear all of Course 4.
Course 6	Clear all of Course 5.
Course 7	Clear all of Course 6.
Course 8	Clear all of Course 7.
Course 9	Clear all of Course 8.
Course 10	Clear all of Course 9.