Opacity Tags generates a new type of tag cloud that uses a link's opacity to indicate its popularity.
This is usually because you're running an older version of WordPress that doesn't have a recent enough version of jQuery. Consider upgrading your WordPress installation or otherwise upgrading your version of jQuery.
Opacity Tags works in Internet Explorer 7 and up, and in modern web browsers. I haven't tested it on Internet Explorer 6 as it's an old, crappy browser that needs to stop being used.
There's a lot that could be preventing Opacity Tags from working. Head to http://www.opacitytags.com and fill out the support request, I'll take a look and respond as soon as possible.