The MapKit card is in the inside the back flap, glued to the cardstock.
Remove the card, and on the back is the access code which you need to enter at: http://alltrails.com/start
You don't need to keep your access code after applying it to your account. Simply login to your account using your email & password to access AllTrails Pro.
NOTE: If you receive an error message of "Invalid access code" when trying to use your MapKit access code, first be sure that you are inputting the code with the dash.