Hi Mia,
in the box which your harmonica came in you will find the product information insert and a separate small red slip of paper with the code.
Below is David Barrett’s reply to the same enquiry a while back in this thread, hope that works for you. If not, you can email him directly and he will assist you: bluesharmonicateacher@gmail.com:
Hello, thank you for contacting us. Here is where you go to start the process to redeem your free month of lessons http://bluesharmonica.com/hohner
The lessons are designed to be studied with an A Harmonica. If you only have a C Harmonica, there is an intro lesson here http://www.bluesharmonica.com/lessons/c_harmonica_intro_lesson . After this lesson, if you decide to continue, you’ll need to pick up an A Harmonica. Best wishes on your studies.