The problem may be due to WinPcap not being installed, since the SnifferReactor depends on it.  The Pion installer attempts to install WinPcap if it doesn't find it already installed, and if it fails, you can't use SnifferReactor.
One thing you can try to see if WinPcap is the problem is to run Pion from the command line instead of as a service, as follows: open a command shell, cd to C:\Program Files (x86)\Atomic Labs\Pion\ and run start_pion.bat.  Now, open http://localhost:8888 and run the wizard again (there is a wizard link near the top if it doesn't start automatically).  This time, if WinPcap is the problem, when you get to the 'Select Capture Devices' page, you should get a dialog box saying something like "This application has failed to start because wpcap.dll was not found."