Election materials should always be returned to your Board of Elections, not your polling place or a third party such as U.S. Vote Foundation.
Your ballot should come with a pre-addressed (though not pre-paid) envelope. If it does not, you can obtain the return address in the Election Official Directory. Choose your State and Region to find the link to your Local Elections Website. You can go to that site for more state specific information.
If your region does not have a website, you will have to contact your local election official, you can find that information in the Election Official Directory as well.