There's a tutorial on Rob Kenny's The Composing Stick with instructions for adding AddThis or ShareThis buttons to your posts. After receiving a request today, I decided to show where the code goes if you're using the Mid-Century Template Set.
How do you add the ShareThis link on your post? I use the same theme, but I don't know how to do it. Can you please put that as a tutorial on your site?
Follow the instructions from The Composing Stick. I might note here that I only added the code to the entry template. When you sign up on ShareThis, they give you the option to customise the look of the ShareThis popup window to match the theme of your site. Look at the CSS for your stylesheet to decide on the link and text colours to use.
After you create the new template module, go to the entry template and look for the following line of code:
<MTEntryTags glue=', '><a href="<$MTTagSearchLink$>" rel="tag"><$MTTagName$></a></MTEntryTags>
Replace with this:
<MTEntryTags glue=', '><a href="<$MTTagSearchLink$>" rel="tag"><$MTTagName$></a></MTEntryTags> | <$mt:include module="ShareThis"$>
Rebuild your blog.