Canon XF100 live stream -- additional hardware needed
You need a capture card.  If you have either USB3 or Thunderbolt, then you can use something like the Black Magic Intensity or Matrox MXO2.  You could also consider a stand alone device that is capable of handling the streaming for you directly, such as the Matrox Monarch or similar devices that don't have to rely on the computer's processing to send the stream.