To highlight text:

Choose Tools > Text Tool.

Select the text you want to highlight.

Open the Tools > Mark Up submenu, and choose one of the following:
Highlight Text: Changes the background color of the selected text.
Strike Through Text: Puts a line through the middle of the selected text.
Underline Text: Puts a line under the selected text.

To change the color of the highlight or underline, choose Tools > Inspector.