the gateway will not except the expiration month and year in the same entry. If you where to input the month and year into the same form element, you would need to hand code a function to separate them back out into individual entries, this is not something we can offer support for.
When the checkout page posts to the confirm page, the form information is stored in session variables. each one will be named "eCartCheckoutForm_[element name]".
On the checkout page, select each form element in design view, In the property Inspector, click the lightning bolt next to Initial Value and select the corresponding session variable