[Ronie]

f you are using a Magic Mouse or Trackpad, you cannot click the Left Mouse button while holding down the Right Mouse button. This means that you cannot aim and fire using only your mouse. Instead, you can aim by holding down the Control (ctrl) key. You'll then be able to fire using the Left Mouse button as normal.
If you prefer, you can change the key bindings for aiming and firing. You can customize your keyboard configuration by selecting Options → Game → Input Mapping from the Main Menu.