My question is nearly an inverse of this one: How do I banish the "You have just inserted an Audio CD." dialog forever?
A long time ago, I selected "Do Nothing" and checked "Always perform this action", but now I have changed my mind and would like Totem to open when I insert a DVD.
If I recall correctly, this setting used to be in the Nautilus preferences, and later in the System Settings, but I have no idea where it is now!