If i understand correctly, you have added a a text filed to your checkout.php page and you are trying to store the value in a session on the confirm page, is that correct?
Once you have done that, on the confirm page, go to the bindings panel and click the + button and select the Form Data (Similar to what you have said in the previous post), and then select the checkout.php. This should add the checkout form from the checkout.php page to your bindings panel, however it will be called something else:
Then your can go to the server behaviors panels and click the + button and select eCart > General > Set Session Variable and from there you can select the new text field.
let me know if you still have any other questions.