If I type the address "site.com/cgi-bin/mt/mt.cgi," even after clearing cookies, I'm not able to add text to, or change text in, an entry, except for a title. However, after clearing cookies and typing "www.site.com/cgi-bin/mt/mt.cgi" then I'm able to create or modify an entry.

This is probably because www.site.com is set as your cgipath in mt-config not site.com as far as cookies are concerned these are different domains.

I had the same problems on my test system if I connected via localhost rather than server-name.

If I take the "www." out of the path and use "site.com/cgibin/mt/mt.cgi" then I can create entries but I'm unable to edit them. On top of that, sometimes I have to log in again after I publish a new entry. As long as I have the "www." in the path and use "www.site.com/cgibin/mt/mt.cgi" then everything isn't an issue.

Open you mt-config.cgi file

Is CGIPATH set to www.site.com/cgibin/mt or site.com/cgibin/mt ?