Ok, here is what you need to do:
1. Output the track you want to compress to one of your RME's outputs.
2. Plug that output from your RME into the 747 compressor's input.
3. Then plug the 747 compressor's output into another one of your RME's inputs.
4. Create a new track in ProTools for the compressed audio and make its input the new input you have used on your RME. Record enable the track to hear the audio signal through the compressor. Then when you are happy with the results you can record your newly compressed audio signal.