You need some kind of "Virtual MIDI cable" to connect Sundog to FL Studio. Unfortunately Windows doesn't ship with such a utility, so you have to download a third party software. I recommend LoopBe1.
Please follow these steps:
Box 1: Select your virtual MIDI cable from the input controller list.
Box 2: Enable it and close the settings.
Box 3: You have to set the FL Studio tracks to different MIDI channels, so that each instrument will respond to separate MIDI note events. Click on one of the small light-boxes to "arm" the track for MIDI input. Click on the same light-box a second time and all tracks should get armed/selected (alternatively you can right-click each light-box separately). FL Studio will automatically assign MIDI channels from 1 to 16 to all selected channels from top to bottom.
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: 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.