Hello, tired. Your 55DU7000 Samsung TV doesn't have an Optical Port. To connect the HW-C400 Soundbar, you must set it up via Bluetooth. To connect your Soundbar to your TV via Bluetooth please follow these steps:
1) Press the PAIR button on the remote to enter BT PAIRING mode. You can also press the SOURCE button on the top panel and then select BT. “BT” changes to “BT PAIRING” in a few seconds automatically or changes to “BT READY” if there is a connect record. If BT READY appears, press and hold the SOURCE button on the top panel of the Soundbar for more than 5 seconds to display BT PAIRING.
2) Select the Bluetooth sound mode on your TV (See your TV's user manual for assistance)
3) Select “[AV] Samsung Soundbar A450 Series” from the list on the TV’s screen. An available Soundbar is indicated with “Need Pairing” or “Paired” on the TV’s Bluetooth device list. To connect the TV to the Soundbar, select the message, and then establish a connection.
When the TV is connected, [TV Name] → “BT” appears on the Soundbar’s front display. You should now hear TV sound from the Soundbar. Once you have connected your soundbar for the first time, use the “BT READY” mode to reconnect.
~ Samsung Solutions