Sorry, it looks like you can only use Betfair in Nevada, Delaware and New Jersey:

"It is legal to bet using Betfair in USA? Yes, so long as you are in the states of Nevada, Delaware and New Jersey via the Betfair owned company TVG. I think all Betfair USA fans are eagerly awaiting the day in which sports betting is legalised throughout the USA and Betfair USA punters can bet and trade as they please."