Make sure that you already have Easy Restaurant Menu Manager for WordPress installed correctly and activated.
Then, after purchasing the widget plugin, unzip the downloaded file.
In the generated folder, there is a readme.txt and a second zip file that contains the actual plugin.
You can use the WordPress plugin upload feature, or unzip the second archive and upload it to your wp-content/plugins/ folder using an FTP program.
Make sure your WordPress install is setup for your timezone. On the WordPress admin, go to Settings -> General and set your local timezone properly. This is required to get the daily specials to switch days in your local time.