Heh, will there is (that's the /usr disk), but you can't see the disk in it. Good point!
So you're right, those are disk slots. Dated reference lost on younger viewers. But hopefully they'll be curious enough to click on them (I'll have some sort of highlight when you mouse over them), and when you do, a little menu will pop up like "Unmount /usr", "Mount Folder", "Mount Disk File", and "New Disk File". These will let you attach /usr and /usr2 to any folder or zip file you like, or create a new zip file.
Part of the reason for all this is that on the Mac, once the app is sandboxed, it can't access any files outside itself that the user hasn't either dropped onto the app icon, or selected through a standard file dialog. So the Mount and New Disk File options are my opportunity to do that.
But even on Windows and Linux, this will be a much more convenient way to get to your files than "name the folder user.minidisk and stick it next to the app before you launch".