There is no master password in the Opera 12 sense - if you give someone access to your Windows account then they can login anywhere you have saved a login for. So don't give anyone access to your account unless you trust them.
If Opera 20 recognized your Opera 12 installation on your system, it should have imported your passwords automatically. Bookmarks - since 20 doesn't have bookmarks in the same sense as 12 did, they were not imported ... but you should find Bookmark Importer enabled in the Tools menu to import them as folders in either speed dial or the quick access bar.
