Once you have logged into the Rain system as an Administrator (your login link should look like 'yourwebsitename.rainadmin.com/admin'), on the far right side of the blue Admin Toolbar, click the gear-shaped button. In the drop-list, click on Settings > Website Settings > Products, you will see a setting for Product Message Below the Add to Cart Button near the bottom of the list. You can enter whatever content you like in there and it will appear there for each of your products. Be sure to scroll to the bottom of the screen and click Save once you're finished entering your content.
Pro Tip: This is a great location to write out your policies on Returns / Exchanges / Refunds.