There's 3 different ways:
At the very top, click "Report Card." Then click "Edit Grades and Comments". You have to hit "Save" after entering comments and before posting.
In Gradebook, click "Options", click on the second tab, then turn on the bottom right toggle for showing posted grades. You get a column that shows if you've posted. You can post the whole class at once... but can't edit comments.
In Gradebook, hover over the letter grade for the term (one of the columns). When you hover, it turns into a clickable link. You get a pop-up that lets you enter comments and post for an individual student. This option will NOT let you post for the entire class, all at once.
What about Advisory?
Changing Grades to Credit/No Credit
Click on Report CardÂ
Go to Students
Then Click on Edit Grades
Change Final Grade to Credit/No Credit