Thought I'd write some simple instructions as it took me a long time to work out how to do this.
I read through about 50 google searches trying to work it out and although many many people have asked this very same question before me, I found the answers to be vague and unhelpful!
In your blogs design screen go to the Layout area.
Then select Add a Gadget any will do (you can move them later where you want them on the page).
From the selection list choose HTML/ Java Script its in the basic menu just scroll down the list a little.
This screen box will appear you wont need a title in the content box paste in the code of the button you wish to use then save.
Move the button where you want it on your page layout and click save arrangement.
Hope this was helpful! Next I'm learning how to put a facebook share / like button on my page wish me luck!