When you have installed and enabled the plugin and you have setup the custom field, you can start using it.
Go to the article you like to add the User Dropdown plugin
You see a new tab. In this example it's called "Joomill Custom Fields" but it's the name you choose for the the Custom Fields Group.
You also see the description if you entered one in the Custom Fields Group.
User Dropdown: Here you select the user or multiple users