Reading and Sending Values to a 104 pc I/O relay board
In the old days, you could hold a scope probe or (my favorite) audible logic probe on the select output of the '138 decode chip, and hear it fire when you hit the right address, or at least hear it fire more frequently if you wrote a tight loop with debug to just read the address over and over again...  but with the CPLD on there the decode signal is probably buried inside.

You could approach the output side first - you can write a program to try toggling the relays at all of the possible addresses.  Though it appears the input and output sides have separate address selection?

You could also test your software against another I/O device, for example reading the status lines of a (true ISA bus hardware, not USB) parallel port, either on the PC104 system if it has one, or a different vintage PC...

You might also simply have a bad board or an address conflict with another module - do try putting the board at a different address.