Before you can record external audio sources all the following conditions must be met:
You must be using at least FL Studio Producer Edition.
An ASIO soundcard driver is selected for your soundcard (press F10 to open the options settings, select Audio and select an ASIO driver).
The recording filter is set to record audio (Right-click the Transport Panel record button), if you are recording to disk (Playlist recording).
A sound source is connected to the soundcard/interface audio input/s (DOH!).
The active soundcard input/s (from 4. above) must be selected on the Mixer track 'IN' menu.
Edison must be set to record audio on the input Mixer track (Edison recording) OR the Mixer track must be armed to record audio to disk (Playlist recording