Attachments have to be less than 1MB. Key 28 is the right key for use with a 044 MPU with a flash card. Be extremely careful when handling the flash card, it's easily damaged by static electricity. Make sure to ground yourself before doing anything to it.
If you just want to change payback percentages DO NOT select the option to clear RAM or EEPROM memory. If you do you will have to completely set up the entire game again. Just select the perform keychip option and then you can change the things you want to. You can't have any credits on the machine to key it, so cash those out before doing anything else.
There isn't a final save and exit option for the keychip menu items. You will have to save most as you go. If you make a mistake just correct it and save it.
You should really look into getting a developer's boot chip for your flash board. It has a built in keychip so you don't have to swap the boot chip with the keychip and also can disable some security checks so your game will boot in just a few seconds. They less you have to handle the flash card the better off you are. The machined socket makes it easy to bend pins too, so take your time an be careful with that. Contact member RB if you're interested in the developers boot chip.