Click on Layout, then Edit HTML
You might want to save the full template before you do anything.
Hit the Expand Widget Templates, then look for the line:
<div class="post-header-line-1">
You want to place the code under this line.
I found the link for the codes here:
http://www.allblogtools.com/tricks-and-hacks/how-to-add-retweet-button-to-blogger-blogspot/
This link allows you to place your button wherever you would like it.
I hope you find this useful, am forever trying to catch up with the times and update my blog!