Bookmarks (1st menu on the left)
There's also a gesture which works on MT devices: two fingers swipe from right to left.
All these things work within a book. If you need/want to get the highlights/annotations of all your books you can use
KoHighlights or export them to json/text/html using the Evernote plugin. (Exporting to evernote itself is broken, so that won't work ATM)