Hi David - when you say you got no response from the TMCC, does that mean you saw nothing at all on your serial monitor window (other than "Echo starting") when you fiddled with the CAB remote?
I'd definitely try the simple echo sketch first, without trying to merge it with the TMCC_snippets sketch. That said, it appears the merged sketch should echo commands from the base, and your connections appear correct.
First possibility: do you have Legacy, or TMCC? The Sparkfun adapter appears to use some ghetto transistor circuit rather than a true MAX232, and since it lacks charge pumps it may need to steal power from the RS232 transmit line.
In particular, I think the TMCC command base does not provide real RS232 voltages (+/- 12V) at the serial port, using 5v and 0v instead. It's possible you need a real MAX232-based adapter rather than the Sparkfun transistor-based device. I've talked to both Legacy and TMCC command bases with the Arduino, but I've only used MAX232-based converter circuits.
Second possibility: RS232 patch cables are notorious for switching the RX and TX lines when you don't think they're doing it. See, e.g., here.