To use iCheck checkbox and radio controls just apply icheck class to any checkbox or radio input and set the control style and color
via HTML attributes data-checkbox="icheckbox_minimal-grey" or data-radio="iradio_flat-grey" and Metronic will take care of the plugin initialization.

This plugins is not included with the core plugins so if you like to use it you just need to include following js and css files on your page:

../../assets/global/plugins/icheck/skins/all.css
../../assets/global/plugins/icheck/icheck.min.js

For more info please chech the plugin's official documentation http://fronteed.com/iCheck/