That's easy to do. I've posted a reply to the button's thread with an alternative button that takes a range:
[Create Multiple Numbered Folders - #2 by Leo)
There are quite a few Learn VBScript / VBScript tutorial websites but I don't know which ones are the best.
If you want to learn scripting in general, not just for doing simple stuff in Opus, then you might also want to look into JavaScript, Python, Ruby or Perl. I use VBScript because people can run buttons that use it without having to install anything but you might prefer one or the other languages or find it has better tutorials and is easier to learn. Different people like different languages. (Personally, I dislike most scripting languages, but they're useful for little things like this.)
You can hook many of the scripting languages up with Opus. (See here.)
I think you can already do that using the Add a Unique Number while renaming rename preset. Set the old name to * and the new name to FolderName_[NUM] (literally) and it will do the rest.
I've zipped up the file now to avoid that problem. Seems like IE7 renames the file but Firefox3 does not, FWIW.
You must be running it in a folder that needs UAC elevation to create folders in it. The button won't work in that situation. (Looks like it tries to do something quite odd, too. Not sure what's going on there.)