Well, since Max5 and Max4 files are compatible with each other you won't need Max5 to run the script, Max4 will do. As it says on the page it's a Max5 exporter. It will convert as much as possible and skip the parts that will flaw. As I understand it the script will be able to convert any Max 4/5 scene files to an earlier version including 2.5.
I can't test the plugin since I haven't got R2.5 and R3.1 isn't installed (won't bother installing it anymore)
I know there are at least a few on this board who use the older versions so it might come in handy?