I needed to do this for the blog that I maintain for Gage's preschool. Parents reported that they couldn't comment so put up a little tutorial - thought I would bring it here.
Under the Choose an Identity column (on the right side of the page, under the actual comment section) and you have given four options. If you have a Google/Blogger account, you can log in under that account name. Same idea for Open ID concerning other blog accounts,(Typepad, etc. for those of you who do not have any blog accounts you will need to choose the Name/URL option. Just type in your name - don't worry about the URL, type your comment and hit the Publish Your Comment button. If you want to remain anonymous, use that option but fair warning - that is the one scary and mean people use. This will work on all Blogger blogs that allow public comments. Hope that helps you and now hopefully, our parents will leave more comments about our school...
Choose an identity
Google/Blogger
OpenID
Name/URL Name
Anonymous