I have Moodle 1.9 I don't know the differences but Grades = Gradebook. Click on Grades in the Administration block. From here I click on the upper-left corner to the "Choose an Action". Here you can select "Edit: Categories and Items" to create you "Items" for putting in the feedback.
1.8 most likely is different so I really can't answer to that but as long as you have an item (or offline item) in the gradebook, you can add feedback which is what you are really after.
One last thought... check YouTube. Put in "Moodle Gradebook 1.8" and you actually will get some video assistance.