I'm not a MAC user, but I would think you could the port the other software is using and type it into the D-RATS add a port box. If it's like Linux, it should be something with a TTY in it. Just don't run the other software at the same time you are running D-RATS or there will be a conflict.
Good luck with your 7100 and D-RATS.
Patrick (N3TSZ)
Patrick Connor