Google has redesigned the Blogger administration interface. As a result, the instructions posted by Blogger a month ago on how to add the "+1" button are no longer valid. "Design > Page Elements" no longer exists. Instead, you need to click "Layout" then click the tiny "Edit" link at the bottom right-hand corner of the "Blog Posts" box. This will give you a list of post option. Make sure the "Show Share Buttons" checkbox is checked, then scroll to the bottom and hit "Save".
Didn't work? That's expected if your blogger has existed for a while (more than 1 year?). Fortunately, this Blogger Templates post on how to add share buttons describes how to hack it (scroll down). This trick worked nicely for all but one of the blogs I administer. Unfortunately, for my wife's Beyond Salmon blog which was created six years ago, it only sort-of worked---the buttons came out very small and the "+1" button didn't appear (even after I moused over the other share buttons, which was the only way "+1" would appear on some other blogs). Bleh.