I have not used it in a while but if i remember correctly, (make sure its all connected and everything) go to Settings > MIDI Settings (something like that) and then you select your MIDI device (sound card) and what port it is (if you don't know what port it is then try 0, if its not 0 then try 1) and then
Go out of that and go to Settings and click on Enable Midi Output and Enable Midi Master Sync and the other enable setting about MIDI
Hope this helps, if you need any other help, holla at me because i used to use FLstudio all the time