1) License key input field is available for the Pro version of URE only. If you have the free version, deactivate it, delete if you wish. Then install the Pro version. Goto the “Settings->User Role Editor” (Network admin->Settings->User Role Editor – for the WordPress multisite) and insert license key there.
2) You may find license key at your personal membership account page https://role-editor.com/membership-account
after login to role-editor.com
3) Pro version is available for download from this link https://role-editor.com/download-plugin/
after login to role-editor.com
4) General description of core functionality included into free version is available here http://shinephp.com/user-role-editor-wordpress-plugin/
5) For description of additional features of Pro version read posts at our blog https://role-editor.com/blog/
6) Some details for Pro version module are available here:
Giving access to the User Role Editor to the single site administrator of your WordPress multisite network, make available for him the selected capabilities only: Look youtube video
If you have any additional questions, please ask. I will try to answer.