Questa dovrebbe essere un copia funzionante.
Sono risalito all'elemento del DOM (il tag "a") partendo dalla selezione.
Ho commentato il codice.

codice:
<html>


<body bgcolor="#BEF781" vlink="#0000FF">
    <center>Digitare le prime tre lettere:</center><br>


    <script>
        function searchForText(toFind, frameToSearch) {
            if (!toFind) {
                window.alert('You have not entered any text to search for');
                return
            }
            if (frameToSearch.focus) {
                frameToSearch.focus()
            }
            if (window.find) {
                if (document.layers) {
                    var ifFound = frameToSearch.find(toFind, !1, !1)
                } else {
                    var ifFound = frameToSearch.find(toFind, !1, !1, !0, !1, !0, !1)
                }
					 //recupero la selezione
					 range=window.getSelection().getRangeAt(0);
					 //dalla selezione risalgo al nodo (<a>) che la contiene e redirigo verso l'url
					 location.href=range.commonAncestorContainer.parentNode.href;
            } else if (frameToSearch.document.body && frameToSearch.document.body.createTextRange) {
                if (frameToSearch.document.selection && frameToSearch.document.selection.type != 'None') {
                    var theRange = frameToSearch.document.selection.createRange();
                    theRange.collapse(!1)
                } else {
                    var theRange = frameToSearch.document.body.createTextRange()
                }
                var ifFound = theRange.findText(toFind);
                if (ifFound) {
                    theRange.select();
						  range=window.getSelection().getRangeAt(0);
						  location.href=range.commonAncestorContainer.parentNode.href;
                }
            } else {
                alert('Please use your browser\'s search facility')
            }
            if (!ifFound) {
                alert('Could not find text:\n' + toFind)
            }
        };
    </script>


    <div align=center>
        <script>
            if ((document.body && document.body.createTextRange) || window.find) {
                document.write('<form action="" onkeyup="if ( this.elements[0].value.length >2  ){searchForText( this.elements[0].value, window );}">' +
                    '<input  style="text-align:center" id="tre" autofocus="autofocus" type="text"  onclick="this.value=null" maxlength="3" size="3"> </form>');
            }
            if (!("autofocus" in document.createElement("input"))) {
                document.getElementById("tre").focus();
            }
        </script>
    </div>


    <center><br><br>
        <a href="http://google.com" class="text">GOOGLE</a><br><br>
        <a href="http://youtube.com" class="text">YOUTUBE</a><br><br>
        <a href="http://facebook.com" class="text">FACEBOOK</a><br><br>
        <a href="http://twitter.com" class="text">TWITTER</a><br><br>
        <a href="http://eccetera.com" class="text">ecc. ecc....</a><br><br>
    </center>
</body>


</html>