Cindy, I had similar problems and found a solution (although it took
me HOURS). Here's a link to the problem and solution. I think
turning on "accept third party cookies" will fix the issue for most.
(For Firefox, that's under Tools-Options-Privacy.) If you continue to
have problems, google "can't comment on Blogger posts" because a few
other issues turned up.
http://www.google.com/support/forum/p/blogger/thread?tid=076dc60a46f69997&hl=en
It is also recommended to set up your own blog for "full screen"
comments to avoid this happening to others.
Alison LeSueur, Media Specialist