This function expects first parameter to be reference to your textarea and second parameter to be the line number

function selectTextareaLine(tarea,lineNum) {

    lineNum--; // array starts at 0

    var lines = tarea.value.split(n);



    // calculate start/end

    var startPos = 0, endPos = tarea.value.length;

    for(var x = 0; x < lines.length; x++) {

        if(x == lineNum) {

            break;

        }

        startPos += (lines[x].length+1);



    }



    var endPos = lines[lineNum].length+startPos;



    // do selection

    // Chrome / Firefox



    if(typeof(tarea.selectionStart) != undefined) {

        tarea.focus();

        tarea.selectionStart = startPos;

        tarea.selectionEnd = endPos;

        return true;

    }



    // IE

    if (document.selection && document.selection.createRange) {

        tarea.focus();

        tarea.select();

        var range = document.selection.createRange();

        range.collapse(true);

        range.moveEnd(character, endPos);

        range.moveStart(character, startPos);

        range.select();

        return true;

    }



    return false;

}

Usage:

 var tarea = document.getElementById('myTextarea');

 selectTextareaLine(tarea,3); // selects line 3

Working example:

http://jsfiddle.net/5enfp/