Then modified a radio button (the schedule)
(action_tile "rb41"
(strcat
"(RB41_FLAG)" "(READ_DIM_FILE)" "(F_NAME)"
); end strcat
); END ACTION TILE
but this does not work
You have started out with code from
www.afralisp.net, I advise you to follow more of their tutorials, you need more Lisp skills to finish this project.
As @BIGAL has pointed out you do not need strcat in the quoted code. But you do have to call the F_NAME function before using the filename in READ_DIM_FILE. And when using strcat to compose the filename the variables used have to contain strings, so you have to either initialize default string values or determine all of them by checking the current state of the dialog.
I personally would not attach actions to individual radios, I prefer to attach them to the radio columns or rows instead. But it seems that the writer of the AfraLISP DCL tutorials was unaware that this is possible.
For more tutorials check out:
https://www.autolisp-exchange.com/