MIDI data can't be recorded through the Line Input (it is used for digital 
audio.) 

MIDI and digital audio are two different kids of signals.  The information 
sent through a MIDI cable is not audio, rather it is MIDI data.  Where 
analog audio that can be recorded through a line or microphone input is like 
a tape recording of a song, MIDI data is like the score or sheet music - it 
holds information about the notes played, but no actual sounds. 

To record MIDI information from a MIDI device, use a sequencer.  

To record MIDI data in .WAV format, select Digital Synth as your recording 
input.