Ideally, you would set up a MSG to read a data table (or part of a data table) from the other PLC. For instance, if you want to open a valve with the ML1100 when a bit in the SLC5/05 is high, you would read the status of the bit in the SLC5/05 and the corresponding bit in the ML1100 would be a condition for the valve output.
To make things easier to troubleshoot, I recommend using the same data table for both PLCs. Example:
Read 1 word from SLC5/05 data table N7:0 to ML1100 data table N7:0. This will read 16 bits from the SLC. If you read 2 elements, it will read 32 bits, etc.)
In order to populate the data table, you will have to re-assign the OTE bit in the SLC to an address within the data table (N7:0/0, for instance). An XIC of N7:0/0 in the ML1100 would be tied to the OTE for the valve.
You can also read "B"-type data files, and probably others. I like "N" files because i can also use them to read integer values, such as pressure, from another PLC