How to buy a Mathzone code online.  

If you have used Mathzone before recently, then your old Mathzone code should still work and you do not have to buy a new one.
Most new textbooks come with Mathzone registration codes  A used textbook will probably not have a working code.
If you don't have a Mathzone registration code then you can purchase a code online for $50 by following these instructions: 

You'll need the Student section code that your instructor will gave you in class.
Go to http://www.mathzone.com/
Click "Students" under "New users register here".
Enter your Student section code.  This will place you in the right course and section.  Click "Next".
Click "Buy online"
Enter the requested information about your school.  Complete the transaction using a credit card.  The code works until  Dec 31, 2012.