I assume you have had a look at the coupon code tutorial here:
http://assets.webassist.com/solutionrecipes/ecart_coupons_sr.pdf
in the tutorl, you create a discount based on the value of the promo code session variable
If session PromoCode = WebAssist
Cart Subtotal * 0.1
you could create another discount rule using another session value for the promo code:
If session PromoCode = abc
Flat rate: 2
to discount 2 dollars