When you are saying that you cannot enter the name of the session variable are you referring to the set session variable interface or the eCart interface for the discount?
It looks like in your code you are setting the session variable promocode. It seems that you are binding this variable to an incorrect value. This variable should be bound to your coupon code text field. Also, you should not have a space in the name of your coupon code text field.
In the eCart discount it looks as though you are not making reference to the promo code session variable.