Person A has $500 Bitstamp USD. Set up a Ripple wallet (https://ripple.com/client). On Bitstamp, go to Withdrawal > Ripple. Follow the link and trust Bitstamp for $500. Fill out the withdrawal page with your details (ripple address, 500 USD). Shortly, you'll have $500 Bitstamp in your wallet.
Person B sends $500 Dwolla. He then goes to Bitstamp > Deposit > Ripple. Bitstamp provides a destination tag -- MAKE SURE TO USE THIS TAG CORRECTLY -- and he sends this to Person A. Person A goes to Ripple > Send, and inputs Bitstamp's ripple address, along with Person B's destination tag and amount/currency. Send 500 USD and in a few minutes, Person B will have 500 USD in their Bitstamp account.
1) PersonA has $500 in Bitstamp
2) PersonA withdrawals $500 to Ripple
3) PersonB deposits $500 to Dwolla
4) PersonB harvests his Bitstamp "destination tag" correctly
5) PersonB posts an ad looking to buy a $500 Bitstamp IOU over Dwolla
6) PersonA answers the add and agrees to sell his $500 Bitstamp IOU to PersonB for $500 over Dwolla
7) PersonB sends $500 to person A's Dwolla address
8) PersonA sends his $500 IOU back to Bitstamp with PersonB's destination tag.
Thanks for clarifying what that destination Tag is for.