You and anyone you collaborate with can add comments to text, objects, charts, and slides. Comments are useful for making notes to yourself, asking questions of reviewers, conveying editorial suggestions, and so on.
You can also highlight text and then add a comment to it.
When you don’t want comments and highlights to be visible, you can hide them.
Commented text uses a color unique to the comment’s author. Any text, objects, and table cells with comments have a comment marker.
Select text or click an object, then click Comment 
in the toolbar.
The first time you add a comment, you’re asked to add a name to use in comments, then a comment appears.
Enter a comment, then click outside the comment to close it.
To read a comment, click it. To remove the comment, click Delete in the comment.
Comments on a slide are always open.
Choose Keynote > Preferences (from the Keynote menu at the top of your computer screen), then click General.
Enter a name in the Author field.
To change the color, choose View > Comments > Author Color (from the View menu at the top of your computer screen), then choose a color.
You can highlight text and then add comments to it.
Select the text, then choose Insert > Highlight (from the Insert menu at the top of your computer screen).
To add a comment, click the highlighted text, then enter a comment.
To remove a highlight, select the text, then choose Edit > Remove Highlights and Comments (from the Edit menu at the top of your computer screen).
Click View 
in the toolbar, then choose either Show Comments or Hide Comments.