From Wix.com

How to enable / add visitor comments to your Wix sites & creations

Go to Widget > Comments
Drag Comments into your page.

To customize the comments further use the property panel (bottom of editor)- see image below

Color: customize the colors of the comment widget’s buttons, text, backgrounds…
Settings: how many comments / page; assign order of comments (first in on top, last in on top)

Save and that’s it. Your site’s visitors can now leave their comments on your page.

How to moderate/delete user comments

Simply open your site in the editor, go to the section with comments and click "delete" on the comment you wish to delete.