If you're wanting to mute while on a call, press the convenience key on the rightside of the phone. To unmute, press it again.
I'll note that this only works if your phone is closed. If your phone is open, unless you've changed the settings for your convenience key, this will bring up the camera.
There is also a selection for mute on the call-screen itself; just navigate over to it with the trackpad and tap to select.