Usually, when you see this headphone model under the sound as an input device and an output device, you simply have to pair the headset to the antenna if you are using it for the first time. Unfortunately, you need to use a Windows computer to do this, but after that it will work under Ubuntu. (I tried the utility under wine, but it didn't detect the device).
Plug the reciever into a Windows computer, and download the pairing utility and start the utility.
Switch the headset on. Then, hold down the Volume up and Mute buttons on the headset until the LED blinks green quickly.
Then, click next on the pairing utility. After a few seconds, it should tell you the headset is ready to use.
When you unplug the receiver from the windows computer and plug it into the Ubuntu, you will have to switch the headphones off and then back on.
Thanks to this thread for the answer that eventually worked for me, and see this page for the pairing directions on the Logitech site.