For elder version I think you have to use find to highlight all occurrences? (not sure)
With newer versions you can use the “Marker” feature:
EmEditor Home – EmEditor Help – Features > Markers
EmEditor currently allows users to set highlight strings. However, in previous versions, there were several limitations,
such as the inconvenience of adding highlight strings temporarily,
that have been rectified in the new version.
The new marker feature allows you to specify strings to highlight temporarily. (…)
The list of markers is displayed in the new Markers toolbar. (…)
The auto marker is a new feature to automatically select text that is the same as the selected text. The Auto Marker tab of the Customize Marker dialog allows you to specify details about the auto marker, such as retrieving the word at the cursor position, matching the word only, or case sensitivity.
With ‘auto marker’ enable, double click a word,
or hold the Ctrl key while clicking on a word,
to ‘Highlights all matches’
“Markers” are highly configurable also. Check it out.