I have a Blue Yeti also (with iMac, but shouldn't be any different).
If you have it plugged in to the USB port, and restart your computer (just in case that makes a difference), it should show under both Output and Input.
Output gives you the ability to attach a wired headphone directly to the Yeti microphone.
Input is of course the microphone input itself.
The mute button on the microphone should be illuminated red if you are properly attached to your computer and it is receiving power. If that is the case then it should be functional. If the mute button is not illuminated red then you are not attached correctly and the microphone is not receiving power from the computer.