Adding a “Report the post” button/form?

To use the plugin in the question you linked to:

  1. Copy the code into notepad or an equivalent program
  2. Save the file using the extension .php (make sure your operating system has file endings turned on so it doesn’t save as filename.php.txt
  3. Put the file into a compressed (or zipped) folder. How you do this depends on your operating system.
  4. Upload the zipped folder to your wordpress blog. (Plugins->add new->upload)
  5. Activate the plugin.