You can export the MIDI file from the app, just click the Export Files button with the piece with the MIDI file selected:
Directly working with the app files from Windows Explorer or the Finder is not recommended, and could potentially cause app crashes or other issues. If you do insist on doing that, you can access the app cache files and recordings directly here:
/Users/YOUR USERNAME/Library/Containers/com.VisualMusicDesign.SeeMusicUnity/Data/Library/Application Support/com.VisualMusicDesign.SeeMusicUnity
To easily get to this folder, use the GO menu at the top of the Finder, and select GO TO FOLDER, and copy that address into it.
C:\Users\YOUR USERNAME\AppData\LocalLow\Visual Music Design\SeeMusic
(This folder might be hidden - you might need to Show Hidden Files in the View options in Windows Explorer)
thank you :)