Adding actual effort

Hi, 

I'm a user with "full access." I have had tasks assigned to me from a certain project. There is an assigned number of work hours, and in the column where I try to enter "actual effort," nothing comes up besides the little lock in the corner (as in, it is not an option that I can enter.) How do I go about this? Even when I go to the timesheet, and the task is there, I can't actually enter hours or actual effort. Any help would be appreciated!

Thanks!