Thanks all for continuing to read and for the responses. It appears that Larry and I have the same issue. I still have not had any success transmitting FSK through the Remoterig boxes, with MMTTY on the control computer, after confirming repeatedly that the configurations of everything is set up correctly. I can key down the radio, but it sends a carrier - no "diddles".
As a work around, I shipped my host a FSK interface cable, and he connected it between the remote K3 and CPU, with MMTTY installed. With Teamviewer, I saw the same issue - I could transmit a carrier, no diddles.
To troubleshoot, we disconnected everything from the K3 ACC port except the FSK interface. FSK then worked, using Teamviewer and MMTTY on the remote computer.
I reconnected the KPA500 in parallel with the FSK interface to the K3 - both worked (I could still send FSK from the remote computer).
I reconnected the Remoterig IO cable to the ACC jack (I have two Elecraft ACC splitters daisy chained), and the problem returned. I can TX, but only a carrier. I see the same symptom if I try sending FSK remotely, through the RR boxes, or through the FSK interface connected between the remote CPU and the remote K3.
It appears that something, either in the Elecraft provided Remoterig box or IO cable assembly that is sinking pin 1 (FSK on the K3) to ground. I can't confirm this yet, but it is apparent the problem has something to do with the remoterig box.
This is where I have come to a stopping point. It seems unlikely that the IO cable is faulty. I suspect that the problem is either a hardware (jumper?) or software (setting?) issue within the RR box.
I think I may need to contact Elecraft customer support since I did purchase the boxes from them directly, pre-configured for Elecraft.
73, Dave