I'm not clear on where you are trying to add the grades--on the Grades page or in the SpeedGrader for that assignment.
If it's the Grades page (gradebook), you should be able to click into the cells in that assignment column for each student and enter a score. If it's the SpeedGrader, you should be able to enter the grade in the provided box.
If these locations aren't accepting entry of values, the first thing to check is the assignment settings, to make sure you don't have "Display grade as" set to Not Graded.
When you say it won't let you enter grades, do you mean there isn't a spot to do so, you get an error message, or something else?
Best,
Suzanne