you wont be able to store the key using the checkout wizard, you need to add an insert record server behavior to the confirm page after running the checkout wizard.
to be honest, this is not a standard use of eCart and can be confusing and difficult to implement if you are not comfortable with PHP and MySQL. Really all I can offer is a general overview of how to accomplish this, not specific steps.
If you are not comfortable with PHP and MySQL, I would suggest posting for assistance in the Job Board.