How to Fix "Mark Occurrences" in Eclipse?

If the option mentioned by laurie is checked and you're wondering why it isn't working, you might have hit Alt-Shift-O sometime:

Cannot highlight all occurrences of a selected word in Eclipse

Note, that "Organize Imports" has Ctrl-Shift-O...

Check the following preference setting:

Preferences > General > Editors > Text Editors > Annotations

In here compare the settings for Occurrences and Write Occurrences.

You probably don't have the Text as higlighted option checked