Just copy/paste this into your local ./conf/userscript.js file.

var default_text = " Enter Keywords to search wiki";
 
function DefaultSearch_blur()
{
        var search_input = document.getElementById('qsearch__in');
        if (search_input !== null && search_input.value == "")
                search_input.value = default_text;
}
 
function DefaultSearch_focus()
{
        var search_input = document.getElementById('qsearch__in');
        if (search_input !== null && search_input.value == default_text)
                search_input.value = "";
}
 
function CheckSearch()
{
        var search_input = document.getElementById('qsearch__in');
        if (search_input !== null && search_input.value == default_text)
                search_input.value = "";
}
 
function installDefaultSearch()
{
        var search_input = document.getElementById('qsearch__in');
        var search_form  = document.getElementById('dw__search');
        if(search_input !== null) {
                addEvent(search_input, 'focus', DefaultSearch_focus);
                addEvent(search_input, 'blur',  DefaultSearch_blur);
                search_input.value = default_text;
        }
        if (search_form !== null) {
                addEvent(search_form, 'click', CheckSearch);
        }
 
 
}
 
addInitEvent(installDefaultSearch);

Can be found in the javascript pages.