You may need to open the Audio properties and ensure that you have selected the right recording device. I see this a lot with these iMac machines that are so popular in classrooms nowadays. It sometimes takes two or three tries to actually discover the correct device as the "default" doesn't seem to work.
Did you try speaking into the microphone for a bit and while doing so watch the volume go up and down? The idea is that you achieve a value that clips into the red only occasionally.