The Diigo Diigolet has some nice features on the iPad. When it is first activated, it will ask you to login (use the same credentials as Diigo.com). The Diigo toolbar will appear at the top of the page. Notice that there are several options available to you–Bookmark–Highlighter–Floating Sticking Note–Comment. The options are all simple to use.
The one that may confuse you is the Highlighter option. To use it, simply tap and hold and then bring the blue overlay over the area that you want highlighted–THEN tap the Highlight button.