I am told that there is a new option in Appearances - Extras: you can enable a Tweet button. I went there, and I saw this:
These are general extras that you can enable for your entire blog.
Enable mShots site previews on this blog
Display a mobile theme when this blog is viewed with a mobile browser
Hide related links on this blog, which means this blog won’t show up on other blogs or get traffic that way
No option to enable a tweet button. Am I just missing it?