3. Check privileges for access to files: data storage (default: gbdb.csv) must have write permissions. If you enabled upload feature, 'upload' directory must have rwx permissions. Other files need only read rights.
If you have uploaded files to a subfolder "/guestbook" of your webserver, then http://yourserver.domain/guestbook/ will open a guestbook. Following the link http://yourserver.domain/guestbook/administration.php will open the administration panel.