If you stored your product key on a previous version of the Office download site and are unable to find it based on how you acquired Office, check your email. If you are an Office 2010 or Office 2007 download customer, you were sent an email message containing the product key when you first set up your account. The email would have come from the domain trymicrosoftoffice.com. If you downloaded Office for Mac 2011, you can contact Support through the How can I get support? the section in the FAQ. You’ll need to provide the email address that you use to sign in to the download site along with your product and purchase information.