This video will show you the two ways in which you can add comments to a google slide.
1. Select the text box you wish to add a comment to
2. Click on the speech bubble at the end of the tool bar
3. Add comment
or
1. Select the text box you wish to add a comment to
2. Click on the speech bubble next to the present button in the top right corner
3. Click on comment
4. Add comment
