The manual grading link for quizzes is currently found in the navigation block under quiz results.
The advanced grading link for assignments is located in the settings block. It would make more sense to have a link in the settings bloc for manual grading or even a link from the quiz results page or both.
There is a request to add the advanced grading option for grading the essay questions (MDL-32804), so it would make sense that the method to grade the essay questions in a quiz would be similar to using the advanced grading method for assignments.