The Isle of Sweet Tarts is an illusive site that is no longer advertised on Neopets. You can get to it by going to http://www.neopets.com/games/sweetarts.phtml. There you will see a page that has four types of sweet tarts. Your quest, as it were, is to collect the four tarts to get to the grand “Isle of Sweet Tarts” (much like a totem for you old style gamers). You must select “begin quest” for this to work.
You can try to figure out the clues on your own (if you refresh the page a few times you will get hints about the mystery locals) or you can go to where I have consistently found the items: