Are you a guest or a host? I am assuming you are a guest as Airbnb does not send invoices out to hosts. In order to contact Airbnb, check your reservation/confirmation and there is a number to contact them there. Also, if you have Twitter, send a message to @AirbnbHelp and they usually get back to you within 10 minutes.
David
Superhost Ambassador ~ Host Club Community Leader ~ Experienced Co-Host