IBAN is the European bank account nomenclature system. In the IBAN/account number box, put your Bank of America account number. When you withdraw, their system will first query the SWIFT/BIC code you provided and because it resolves to a non-IBAN institution, the account number you provided will be accepted.

I have withdrawn countless times to my non-IBAN account so I know this is definitely the solution. Here, have a look at mine:

http://imgur.com/3enWZpI

Perhaps it's better to contact Bitstamp support, I'm sure they know what you need