Why not just use the addthis module rather than adding a block. If you however want to add a block, just go to admin/structure/block click on add block. Paste your script in and select filter type of php code and click save
If you do not have PHP filter in the filter type, enable it under the core module
To attach CSS or JS files to a specific block you will need to build a custom module that implements the hook_block_view_alter to manipulate the block render array and then use Drupal 7 FAPI's #attached attribute to attach the file to the block.