From README:
New files that must be copied over:
...
admin/includes/boxes/returns.php
admin/includes/classes/returns.php
...
And in .zip, there is no such files. Shall I assume that "admin/returns.php" is the same file? And if so, should I symlink or copy? If that's the solution, I have another problem instead... I get errors about the declaration of tep_db_connect, in admin/includes/functions/database.php.