The library is well documented and adding the Annotator is quite easy. Everything is done in two steps, first downloading the Annotator library and second add it to your page alongside jQuery (yes, jQuery is required for the Annotator to work). There is also the option for adding plugins to extend the functionality, plugins such as user accounts, tags, filtering or formatting. More about the library can be found on the documentation page.
Also, the popup that provide user information and comments can be easily styled via CSS.