All,
Somehow, I have managed to disable 'variable/ element highlighting' in Aptana Studio 3.
Online search brought up someone's advice to enable 'Highlight current line option in the General > Editors > Text Editors section of the Preferences dialog', which is already enabled and I think is actually for line highlight, not variable highlight.
Thanks,
Since last release it's turned off by default, to activate it click on little marker icon on toolbar.
Found answer: Minor Highlighting bugs on Mac, see comments:
As noted on attached ticket, we've turned off mark occurrences by default until we are able to make some performance improvements.
If you want it on, you can turn it on by clicking the Hightlight Pen icon in the toolbar == shortcut key Alt-Shift-O on Windows.