I can't double click to create a student comment.
You may find that your cursor leaps across the PowerGrade spreadsheet when you double click on the number to the left of the student name. There is an easy way around this annoyance:

-Mac users: hold down the Apple key and SINGLE click on the number. The Comment window will open.
-Windows users: hold down the control key (CTRL) and SINGLE click on the number. The Comment window will open.

How to add a student comment.
How to create a custom comment.