If you currently have free downloads available on Last.fm then these will be available on MP3.com too. This will happen automatically, so you won't need to alter any settings. In the future, you'll see an MP3.com opt-in button within the Music Manager for all the tracks you upload.

If you'd prefer not to have any of your free downloads featured on MP3.com, you can opt out completely here: http://musicmanager.last.fm/account/preferences