First you need to be sure you have the proper extension version for your 1.2.7 install
http://marketplace.abantecart.com/marketing/addthisIt should be available in your account for download of whichever version you need. If not - you should contact the Marketplace for help.
If you have issues with an extension - you should always contact the extension developer - there are several ways to do that- all are in the Market Place. Look under the Support tab on the extension. Or post a comment and the developers get those in an email to respond to.
Always tell the developer what other extensions you have enabled as there are sometimes conflicts - for Add This there is a known conflict with the Lightbox extension, however if you contact them they have a solution already made.