You will need a "virtual MIDI cable" to connect Sundog to FL Studio.
Windows: Please follow the guide here: http://feelyoursound.com/setup-midi-windows/
macOS: Please follow the guide here: http://feelyoursound.com/setup-midi-os-x/
Basically, you set the MIDI Out of Sundog to the virtual MIDI cable, and the MIDI In of FL Studio to the virtual MIDI cable as well. See the example for FL Studio on Windows below (the virtual MIDI cable is called LoopBe here).
You can also watch this short instructional video:
Important: You will have to set all instrument numbers inside Sundog to "Off" (the default value)! Otherwise FL Studio will get confused by the different instruments and deselect the light-boxes.
Tip for Windows: I recommend to use ASIO as audio output device. FL Studio seems to have MIDI timing issues on some computers when you use a different sound driver.
Please make sure that you don't create a nasty feedback loop with your MIDI connections. FL Studio should not send MIDI notes back to Sundog! Otherwise it might happen that these notes will trigger Sundog to instantly send new notes to FL Studio as well, and the whole circle starts again.
If you want to use an external MIDI keyboard with your setup you should keep these things in mind:
1) Set Sundog's MIDI In to your external MIDI keyboard
2) Don't use your external MIDI keyboard as an input device in FL Studio
Reason: If you use the keyboard in both Sundog and your DAW you will get multiple MIDI note events in your DAW as soon as you press a key. One time from the keyboard itself, the other time when Sundog sends its notes.
I hope this small guide was useful! If you have further questions (or found an error) you can contact me here.