The latest Google Reader update detached all the social features, including the section that allowed you to read the items you've joint. Fortunately, the shared items page is still obtainable at http://www.google.com/reader/shared/username (replace username with your Gmail username) and you can pledge to this page in Google Reader. Click "subscribe" and paste the URL of the shared items page. If you don't have a Gmail account, load the shared items feed in Reader, right-click "Your shared items" and copy the URL.
You can now use Google Reader's search box to find a post you've joint. Click the "All items" drop-down next to the search box, scroll down to the end of the list (or just press "End") and you'll discover the shared items feed.
To unsubscribe from this feed, you require to go to the settings page, click "Subscriptions", type "shared items" in the search box and click the "unsubscribe button".