although you can use the so called AddThis.com "Feed Widget" pasting its code to one of a .com sidebar "Text Widget" (but just don't forget to include your Feed URL and optionally 'Blog Title' there), the "Bookmarking widget" (which allows to bookmark any particular post of your blag ) does not work here and now without the same kind of an internal support as was made for the 'Digg This' button.
a mere embedding an image (as in all examples above) just gives you a clickable image link to the AddThis service without actually telling it which post (or feed) needs to be bookmarked, what is probably not exactly what you originally intend.
the correct code for the "Feed Widget" only is:
<a href="http://www.addthis.com/feed.php?&h1=http://YOUR_BLAGNAME.wordpress.com/feed" title="Subscribe using any feed reader!"><img src="http://s3.addthis.com/button1-rss.gif" title="AddThis Feed Button" /></a>
as already was suggested by Adam another cool aproach is to use a very nice Alex King's plugin for WP called 'Share This'. you can see it in action on the Alex King's own blog or on this one.
[kudos to Matt who took into consideration and Ryan who has finally implemented a remedy to a "Full text feed problem when using <!--more--> tag" real issue.]