I had the same problem with my windows edition: I selected the text with my mouse and then right clicked and the options that appeared indicated that highlighting was an option, but it was grayed out as if it was not available. After a bit of research, I found the answer.
Answer: Select the text and use the following shortcut: CTRL + M.
Incidentally, after using this shortcut, I found that the option to select the text and add a highlight via the 'right click' drop down menu became available.
The next question I had was: How do I delete this highlight as there was no apparent option.
Answer: Its in the Navigation Pane.
Open the navigation pane. At the top of the navigation pane (above the actual list of contents) there are 3 buttons which control what content is displayed: select the middle button, which is for bookmarks (this menu also displays your highlighted text). Next, find your highlight from the list and select it. Click on the button with a gear on it at the top of the navigation pane. A drop down menu with the option to "delete selected" should appear: click it.