How can I highlight code with ACE editor?

First you want to declare your line number as a global variable.

var erroneousLine;

This is the highlightError function, which takes in a line number (lineNumber) as its parameter. which could be triggered from an error message or using editor.selection.getCursor().row to get the current row, or something else.

function highlightError(lineNumber) {
  unhighlightError();
  var Range = ace.require("ace/range").Range
  erroneousLine = editor.session.addMarker(new Range(lineNumber, 0, lineNumber, 144), "errorHighlight", "fullLine");
}

Notice that I have declared a errorHighlight, which is how this will be highlighted. In your css place the following:

.errorHighlight{
  position:absolute;
  z-index:20;
  background-color:#F4B9B7;
}

This function unhighlights the already highlighted line

function unhighlightError(){
  editor.getSession().removeMarker(erroneousLine);
}

Highlight the word:

var range = new Range(rowStart, columnStart, rowEnd, columnEnd);
var marker = editor.getSession().addMarker(range,"ace_selected_word", "text");

Remove the highlighted word:

editor.getSession().removeMarker(marker);

Highlight the line:

editor.getSession().addMarker(range,"ace_active_line","background");

There is a server side version of the highlighter (which runs in node.js) available, that'll probably be portable to webbased javascript fairly easy.