This is simple it's just one file but back up everything anyway.
If you want to place the code you generate off the AddThis site between the Product Price and the Product Description download your tpl_product_info_display.php in your custom template -> template folder open it into a text editor and find the following code.
<!--eof free ship icon -->
<!--bof Product description -->
Place the generated code between those two, eg:
<!--eof free ship icon -->
(Add This Code)
<!--bof Product description -->
If you want it anywhere else, look for the code name for the form i.e. Product Price Block and place the code between an !--eof and a !--bof, (as above). You can try out different places to see what suits your site.
Then upload the saved file to your custom templates -> templates folder
What's confusing you on FTP?