Hi Dave, there are some instructions on how to install plug-ins here:
http://moi3d.com/faq#Q:_How_do_I_install_a_plug-in_script.3F

Some things to look out for - make sure you have unzipped it, you need to copy the contents of the .zip file into the commands folder, not the .zip file itself.

Then to trigger it set up a shortcut key with _Subdiv_beta9 as the command name.

- Michael