WP-Cumulus displays your tags and/or categories in 3D by placing them on a rotating sphere.
There are a number of things that may prevent WP-Cumulus from displaying or cause it to display a short message about how it needs the Flash plugin.
I'm not sure how beneficial tag clouds are when it comes to SEO, but just in case WP Cumulus outputs the regular tag cloud (and/or categories listing) for non-flash users. This means that search engines will see the same links. They're hidden through CSS by default, but there's an options to make them visible.
As of version 1.12 the source code is available from wordpress.org under the GPL license. Click "other versions" and get the developer version.
If this happens you should change the aspect for the movie to make it wider. This can be done by increasing the width, but also by decreasing the height. Both will make the movie 'more landscape' giving long tags more room.
Because of the way Flash handles text, only Latin characters are supported in the current version. This is due to a limitation where in order to be able to animate text fields smoothly the glyphs need to be embedded in the movie. The Flash movie's source code is available for download through Subversion. Doing so will allow you to create a version for your language. There's a text field in the root of the movie that you can use to embed more characters. If you change to another font, you'll need to edit the Tag class as well.
More info here.
This is usually caused by a Flash security feature that affects movies served from another domain as the surrounding page. If your blog is http://yourblog.com, but you have http://www.yourblog.com listed as the 'WordPress address' under Settings -> General this issue can occur. In this case you should adjust this setting to match your blog's actual URL. If you haven't already, I recommend you decide on a single URL for your blog and redirect visitors using other options. This will increase your search engine ranking and in the process help solve this issue :).
This list is far from complete. Chances are you'll find the port you need through search engines. If you're having trouble setting it up, please contact the author(s) of the port.