Here is a link to a package of files.
Unzip it, and it includes some python scripts, a test story so you can see if the conversion is working on your system, and a README that tells you the command line to use. If you have python installed, it should just work, so try copying and pasting the command for the test story "Another Day at the MIA". If it works, the hard part is done!
For your own stories, create them in Twine just like mentioned in the README, and export the source as a text files (ending in .txt). Then following the same instructions as the test story, you can convert your own, turning it into a .bin file.
You can put those bin files on a micro-sd card, insert that into the Choosatron, and DURING power up (while you plug in the power) hold the 2 button, and it should load stories from the memory card. I've made this easier in newer firmware. If you loaded the firmware yourself, I can provide you a newer version if you like to improve it a bit.
This is an old method I've hacked together since the beginning, so it wasn't technically meant for other people to use, but I've tried to simplify it as much as possible so it can at least work! Let me know if you have questions, but I hope this at least lets you move forward. Good luck!