In Google Sites, when you want to allow comments, you will have to enable them on each page. You cannot enable comments just for the site and have them show up on the corresponding pages.
Open that page that you want to enable comments on, and click Edit on the top menu bar. Click on the More Actions button and a drop down will appear. In the drop down menu, click on Page Settings, and check the box that is next to Allow comments. Once the box is checked, click on Save and comments will now be enabled for that page.