My first reply really applies to the first visualizer you linked to, which does need to be installed using their installer.
Other plugins should show up with a lego-block icon with iTunes notes on one face and an atom on another. These files can be moved directly to ~Library/iTunes/iTunes Plug-ins. If you are not seeing the lego-block icon, again, you need to download and install Stuffit.
(I tried editing my earlier post, but by the time I finished making my changes, the editing time was up and I couldn't save my changes.)