since the code wrapper gadget is gone, here's what I'm doing now:

1. go to disqus.com, and sign up for an account
2. copy your account "shortname"
3. in your google site, insert a gadget
4. search for disqus
5. insert the one named "Disqus comments"
6. in the Disqus Shortname box, paste the shortname you copied earlier
7. adjust the rest of the gadget as desired
8. click OK, save your page, and enjoy!