Highlighting and Annotation tools are not built into the web app.  However, since it is just a web page, you can use any one of the many highlighting and annotation tools/services already available.

One of our favorites for its simplicity is http://www.awesomehighlighter.com/ (just enter the URL of the Ryuzine you want to highlight or annotate).  We also like this one because it doesn't mess with the Ryuzine scripts, allowing the app to still function correctly.

However there are many others available to try, check out this list!