According to many people who work for Adobe, they only have the 3 card options: Visa/Express/Master
I too have only a Discover credit card, and am suck here very disappointed in Adobe for not thinking the payment process fully.
Perhaps in due time, there will be another option for not just Discover card holders, but to everyone else who is left out.
The thing is it could be the company that processes the sales for Adobe. Most companies use a third party for credit card purchases. If that company only offers visa, mastercard and AMEx, then not much Adobe can do about it other than find another company, but if the price is right, not much of a motivator to switch companies.
That said, I am only assuming, they may be doing all sales in house and just never thought about the discover card.
You could see if your bank can cash the card, take the cash and put in a pre-paid visa card. Most banks, gas stations, and major stores sell pre-paid visa cards.