As far as I know there is no button for this in Google Reader. However, Google Reader has the nice “Send to” feature.

By adding Google Bookmarks as a custom “Send to”, you can simply press “Shift+T” to save whatever you are watching to Google Bookmarks.

To create a custom “Send to” click “Settings” then “Reader Settings” (Upper left corner.

Go to the “Send to” tab. And on the bottom of the page, click “Create a custom link”

Enter “Google Bookmarks” as the name.

Enter “http://www.google.com/bookmarks/mark?op=edit&output=popup&bkmk=${url}&title=${title}” as the URL.

And enter “http://www.google.com/favicon.ico” as the Icon URL.

Press save.

Now youre done! Simply press “Shift+T” while reading something in your Reader to add it to Bookmarks.

Note: If it isnt working, you may have an issue with popup-blocking. Sending to Google Bookmarks opens a popup windows.