The messages are in the language_text table of the database, rather than a file, and can be edited using the Editor in the Admin's Language Manager. Search for the terms you want to modify.
There is not a way to export out the entire member registration in ATutor itself, though you probably could export it as SQL from the database with a tool like phpMyAdmin, and import that into your new site the same way. Or you can export out a class list from each class and import those into the equivalent course on the new site.
Use the Manage>Backup tool to export or import course backups as an instructor, or use the Courses>Backups tool as the administrator to do the same.