When making a meeting request, I've noticed that the Add Public Folder command under Add Others is disabled. How can I make it active?

That command doesn't work in meeting requests. It's active only when you're adding a public folder to a group schedule in Outlook 2002 or later so that you can check the group schedule's free/busy times. You can, however, invite a mail-enabled public folder to your meeting by selecting it from the address book.