Log in to your Google Site.
Select the page that you wish to place your comment box in.
Find & Click "Edit Page"
Find and Click "Insert", then select "More Gadgets".
In the "Search Box", enter "Code Wrapper" and select the "Code Wrapper" gadget.
After selecting the "Code Wrapper" gadget, a "Set Up Your Gadget" Box will appear. Under "Embed Code" copy and paste the HTML Code found in this link...
After you copy and paste the code from the link above, you may choose to change the "Display" found on the bottom of that same "Set Up Your Gadget" box. Click "OK" when you are done.
Your new gadget will appear on your page. You may want to edit it by right clicking on the gadget to change it's position on the page, it's properties, or to remove it. Some of the "Properties" that I find useful to change are under "Display". You may choose to include a scrollbar on the gadget when necessary. You may change the title of the gadget from "Code Wrapper" to whatever you like. You may also change the height and width of the gadget.