It you can be more specific, I can probably help. Most Omron rack mount type PLC's require registration in the I/O table. What type are you dealing with?
Required information
Model of PLC processor
Rack layout (from the left)
for example CS1G-CPU43E, 16 point input model CS1W-ID211, 64 point output module model CS1W-OD261 etc etc.
They are all just numbers, no %, I, Q, X, or Y.
This can be very cofusing to a newbie to Omron but is really appreciated by us oldies.