A tally light is a small signal-lamp typically seen on top of a camera. It is usually located above the lens or on the electronic viewfinder and communicates that the camera is 'live' - i.e. its signal is being seen in Program at that moment.
Tally light systems by MetaSETZ (http://www.metasetz.com) are currently the only systems supported by Livestream Studio.
Once you have acquired your Tally Light system, connect it to your computer via an available USB port. The system should recognize it and automatically install the drivers.
Open the Livestream Studio software and open Settings: Advanced.
Under Other, check the Tally Lights option. Save your settings.
Go to the Inputs tab and select the local input to which you want to assign a tally port, then click the corresponding gearwheel to the right to enter that input's settings.
Select Tally and assign one of the available tally ports to your camera as seen below. Do this for each of your cameras.
When the camera is set to Program, the selected Tally Light will turn on.