You have to follow the instructions:

• Download and unzip the extension. LINK: https://github.com/armandocat/steemit-more-info/archive/master.zip
• If you are using Chrome
  • Open the Chrome Extentions page by typing chrome://extensions in your Chrome Browser.
  • Drag and drop the root folder of the project you just unzipped (steemit-more-info) inside the chrome extension page.
• If you are using Firefox
  • Open about:debugging in Firefox, click "Load Temporary Add-on" and select any file in the extension's directory.
• Go to Steemit!

For chrome, you have to drag and drop the directory, not the zip.
For Firefox, you have to load a file, for instance the file manifest.json that is inside the folder, not the directory or the download directory