I published an article on computer-aided roll organ arranging for crank
organs in the latest (April) Carousel Organ Journal of the COAA. While
most of it is concerned with the mechanics of punching rolls (which may
or may not be needed for your Alderman 26er), I do discuss the use of
computer software, such as Cakewalk.
You need to tell your program what notes your organ can play, and
find out how to make the program check your arrangement for illegal
(un-equipped) notes that your organ doesn't have. Others have said
that Cakewalk can do this.
But first, figure out what musical key(s) your organ plays best in
(depending on which "black" notes it has), and its range from high to
low. Then transpose the given MIDI piece into those keys and see how
they fit the range, especially the melody. Also beware of chords that
your bass section can't support well.
Finally, move out-of-range and un-equipped notes to notes your organ
Play your piece, using appropriate "stops" like Pan Flute, Bottle Blow,
Ocarina, and Flute. Make sure any repeated notes have enough silence
between them so your organ valves can close and re-open. When in
doubt, shorten or delete notes -- simple is better.
I have very little experience with Cakewalk, since I use my own
program, but everyone says it is the lowest cost and easiest to use of
such programs. I'd advise you to just play with it for a while, trying
things out and gradually getting used to it. This usually works better
with a complex program than trying to find out how to do just a few
You will have to learn how to do many things anyway -- moving notes
up and down in pitch, changing their durations, transposing -- so just
jump in and have fun, and pretty soon you'll be able to apply Cakewalk
to your organ work. Best wishes