Error opening serial port '/dev/cu.Bluetooth-Modem'. (Port busy)
You are not selecting the correct port to upload to - hence the strange message about the Bluetooth modem. Start by unplugging your Uno and running Arduino. Select Tools -> Port from the menu bar. You should see a list of options. Now plug in your Uno and do the same again. You should see an extra port listed now - this will be the Uno. Select it. You will need to check this every time you connect a board. 

Also, before you click Upload, make sure you have Arduino Uno selected under Tools -> Board in the menu bar.