Had a hell of a time getting Buzz to let me add my Tumblr as a source. Directions found a few places around the web tell you that you must associate your Tumblr page with your Google profile by adding this tag to your page:
<link rel=”me” type=”text/html” href=”http://www.google.com/profiles/[YOUR GOOGLE USER NAME]”/>
If that doesn’t work for you after a day or two, you can use the SGAPI Recrawl Tool to get Google to recrawl your site and pick up the link. Then you can add your Tumblr page as a “connected site” and Buzz will syndicate your posts.