To enable backup for a company, ensure that you've already added the company to ChronoBooks. (You can do that by selecting the "Add a Company"/"Connect to QuickBooks" button(s) on the Companies page.)
Go to the Companies page and select "Enable Backup" next to the company that you'd like to back up. You'll then see the company's first backup.
Remember that manual backups are not normally necessary, as ChronoBooks automatically records new changes 5-10 minutes after they're made in QuickBooks Online. We also run one full backup each day.