To add your microphone, simply click the green "Add Layer" button, select the Audio Source option, and then select the microphone you'd like to add to the scene.
Please note that you will need to add your microphone to each scene in which you want to be heard, and anything listed in the "Audio Mixer" at the bottom left of your Lightstream editor will play its audio when live.
High Fidelity Audio
If you're wanting to capture high fidelity audio with your source, then you'll want to check the "Optimize for music" option.