You will need your Envato Purchase Code for the installation. You can get it by following this guide.
Download the latest package zip file from CodeCanyon. You will find "SMS.Gateway.Package.zip" in the "Package" folder after you extract the zip file you downloaded. Upload it into the "htdocs", "httpdocs", "public_html" or your domain folder. Now extract the zip file into the same folder.
Now you should be able to start the installation process by visiting your domain or subdomain URL using web browser.
You will see following page when you load it for the first time. Enter appropriate information in the form, select the time zone you want and click on "Install.
Note :- User you will create on installation gets an Admin role. As an Admin you can create new users. Those users can login using their mobile and use the SMS Gateway functionality. As an Admin user you can see all the messages your users sent.