...
This plugin utilize Zen Cart new feature for encapsulated plugins for easy installation without manually copying folders and files. The simple process is as below.
Download zip file from SMSto - Zen Cart Plugins (zen-cart.com) and extract
Copy “smsto” folder under the server public directory “zc_plugins”. The end result would be for example “public_html/zc_plugins/smsto”
Login to the administration Admin area of the site
Navigate to Modules → Plugin Manager
You should see SMSto plugin there. Just click on it and press button Install
...