You need to change some code in 2 files for that, the common.js file & product.tpl file.
Open & edit the catalog>>view>>>javascript>>common.js file & find...
if (json['success']) {
$('#notification').html('<div class="attention" style="display: none;">' + json['success'] + '<img src="catalog/view/theme/default/image/close.png" alt="" class="close" /></div>');
$('.attention').fadeIn('slow');
& change the 2 attention to success & there are 3 places to change this in the common.js file.
Then open catalog>>view>>>theme>>default>>template>>product>>product.tpl file & find...
if (json['success']) {
$('#notification').html('<div class="attention" style="display: none;">' + json['success'] + '<img src="catalog/view/theme/default/image/close.png" alt="" class="close" /></div>');
$('.attention').fadeIn('slow');
& again, replace the 2 attention with success & thats it
