This 'report' is actually a tool like the standard Manual grading quiz report,
but which lets you grade all the responses by one student, rather than all
the responses to one question.
Once the plugin is installed, you can access the functionality by going to
Reports -> Manual grading by student in the Quiz adminstration block.
To avoid confusion, you may wish to use Moodle's
to rename the standard 'Manual grading' report to 'Manual grading by question'.
The string you are looking for is in the 'quiz_grading.php' component. You need
to edit the 'grades' string.