I encourage you first to contact the person who gave you the code and making sure you have the correct code.  Tell them the exact message you are getting when you enter it.

If they say it should work, I suggest using the "Report a Problem" link (it's in the upper right of the window for me) to contact KA staff directly, including the class code you are trying to use.  They may be able to tell you if it is a valid code, or help you figure out what's going wrong.

~ Janice, KA user