You have not provided the relevant info. Post a link to the datasheet for the keyboard. The acronyms DO/DI are not usually used with serial communications.
I remain of the view that this is not an RS232 device.
As well as needing information about how to make the electrical connections (which is not obvious from what you have posted) you will need information about the format of the data sent by the unit.
Or do you have the electrical and programming skills to create an Arduino program that can be used to figure out how the thing works?