You can access the manually marked tests (Essay, Short Answer and File Response question types) through the grade Centre.
Go into the Grade Centre and select the Column which the test is associated with,then select the cell where the student's Test is recorded, click on the small chevron on the right side of the cell.
Once that drops down select 'Attempt (dd/mm/yy)' and it will come up with the student's answers.
If you have associated a Rubric with the questions you can open that to help you mark.
If the Associated Rubric is a marking rubric, then you can mark the questions individually using the rubric.
Remember to Press Save before you move onto the Next Student.