Subversion is a version control system. It allows you to track the history of your source code and makes collaboration much easier, since people can submit changes, get updates, and resolve conflicts regardless of their physical location. Version control is an invaluable tool in a programmer's toolbox and is definitely something you should learn more about. http://svnbook.red-bean.com is an excellent reference. You only need to read the first couple of chapters to get a handle on the basics.
Many rails plugins do require subversion to install, but this one doesn't. If the URL begins with svn://, then subversion is required.
You should run script/plugin from the root of the directory that was generated by the rails command, not in the app directory. That is, when you type ls or dir (depending on your operating system), you should see app, config, lib, log, and so forth. If, after running the plugin command, you get something like "ruby: No such file or directory -- script/plugin (LoadError)", then you're in the wrong place.
The command you tried should work if you're running it from the correct place. If it doesn't, I suspect there's probably a network issue of some sort. Are you behind a proxy? Give us the exact error message and someone can probably help you.
