The SU is the building next the bus stop it's got a bIg "SU" banner on it. If you follow the footpath towards the stadium going down the back of it you will see the boards saying "Supporters club reception" and the ATM (free).
If people stayed on the bus until the Brighton University at top of the hill. It's only a very short walk down the hill which takes you to the west stand.
Not only that, the supporters club reception is in the back of the Student Union building and there is an ATM too.