Login with your credentials. Otherwise, as a Participant you will choose the option Enter as a Guest. Type your full name and click Enter Room.
Initially the only icons you will have are a Speaker and Set Status icon. Once the host promotes you to a Presenter role, you will see icons for microphone and video appear. Click on the drop down arrow next to the microphone icon and click Connect My Audio. (You may not have to do this step, if you see the window pop-up that you see in the next picture). Both icons will change color from gray to green to indicate they have been enabled.