The importer and exporter use your device's Vogel CMI Documents directory. You can see the contents of this in iTunes > DEVICES > [your device] > Apps > File Sharing > Vogel CMI > Vogel CMI Documents. You can drag files in and out of this directory while your device is connected to iTunes.
You can export a song as a MIDI file, and import MIDI files into a song. The settings used in MIDI import and export are defined in Page 3, the Instrument page.
Each instrument voice can be associated with a MIDI channel and a General MIDI instrument. Both these settings are used when exporting a song as a MIDI file, but only the MIDI channel number is used on import. Choosing a General MIDI instrument is your best chance of having the exported MIDI file sound a little like your composition when played by normal MIDI players.
You can use any of the 16 MIDI channels for each instrument voice, but you should probably avoid channel 10, which is used for percussion with a different instrument on each note. If you have multiple instrument voices each with the same voice, you can choose the same MIDI channel for all of them, but be careful that you don't have two or more notes with the same pitch overlapping or they will interfere with one other.
After you have set up the MIDI settings for each instrument voice, you can export the song. From page R, click on the song settings button (top left button with the name of the song) then click on the "Export Song" button on the bottom right. Check at the bottom of the screen that your MIDI channels are set up correctly. Then click the "MIDI file: save as [song name].mid" button. A MIDI file with [song name].mid will be stored in your Vogel CMI documents directory (see above) and will overwrite any file of the same name without asking for confirmation.
Before importing a MIDI file, make sure that you have set up an instrument set with the MIDI channel numbers you want to use, and that it is selected and highlighted in Page 2.
On Page 2, tap the [+] button at the bottom of the list of songs. Select the "New song from file" option, then choose a song from the list of .mid files in your Vogel CMI documents directory (see above). Tap the "Import [song name].mid" button and a new song will be created from the contents of the MIDI file. This may take quite a while, and the floppy disk might get tired and stop clunking, but please wait until the completion message appears.
Note that due to the CMI architecture, which uses eight monophonic channel cards, there are some significant limitations on MIDI import:
The importer will do its best to handle polyphonic input. It will put a polyphonic note (that is, a note that overlaps another note with the same voice) into the first empty instrument voice with that channel number. If they are all full, and the new note has the same pitch as a previous note (unlikely), it will stop the previous note and insert the new note there and you will get a message. Otherwise the imported note will be ignored and you will get a message.
Note that General MIDI and other similar meta-events are ignored.