first, what you want to do?
second, guides are on forum.doom9.org
third, that error means you fed the avs script to the audio input but obviously the avs script had NO AUDIO.
fourth, before encoding, demux the audio from your mp4 file (with mp4box or yamb) to get your audio stream to encode. as an alternative you can use the DGIndexNV (if you have an nvidia card AND you have a license) that will both index the mp4 and demux the audio track.
fifth, if you didnt read the guides and dont know what all this stuff means, megui is probably not the software you are looking for.