Are you choosing the DTS track from the DVD menu (usually under 'Audio' but sometimes inexplicably under 'Languages')?
If you choose the DTS track from the DVD menu and the Yamaha is set to Auto for the input to which the DVD player is connected, it should identify the signal as DTS and automatically choose the DTS decoder to play it. You MUST be using a digital connection (either optical or coax) from the DVD player to the receiver.