Some of you may already be ahead of this, so please bear with me.
The first thing to try is this…
1. Go to your Creative Cloud desktop app (which you were prompted to install when you subscribed to CC)
2. Go to 'Files' and click on 'start syncing'
3. Give it some time to start syncing some of your media as needed… you can click on 'Open Folder' to automatically go to it's folder and verify its location.
4. Dismiss the Preference>Auto Save dialog and re-open it (this should un-gray out the checkbox for you)
5. In Preferences>Auto Save, check on 'save backup project to creative cloud'
Expected result: If you don't have any previous auto saves on your cloud files, nothing will happen at first because there is no new auto save to copy up there yet. As soon as Pr gives you a new auto save file (which requires the set interval (15 min by default) to pass by and a savable change to happen) that should be copied up to your CC Files into a folder path like this: CC Files>Premiere Pro>8.0>Adobe Premiere Pro Auto-Save