Tagboards are similar to guestbooks. They let users comment on your whole blog not just one entry. Unlike a guestbook, they allow for a running conversation on your blog homepage. Follow these steps to add a tagboard to your Blogger blog.
Step 1:
Sign up for a free tagboard account through the service of your choice. Enter “tagboard” into your favorite search engine to find a free service.

Step 2:
Generate the HTML code for your tagboard. Copy the code.

Step 3:
Log into your Blogger account and go to “Dashboard.” Select “Templates.” Then select “Edit HTML.”

Step 4:
Scroll through your template to the spot where you want to add your tagboard. Paste the HTML code.

Step 5:
Click “Save Template” to keep the changes and apply them to your blog. Republish if necessary.

Source: http://www.ehow.com/how_2019506_tagboard-blogger-blog.html