I always include DEFINE HSER_CLROERR 1 if using HSERIN just to be sure it' never locked up on me, but I'm not sure that's the problem.

Try sending first. Something like HSEROUT ["Hello World",13,10] to MCS terminal. If this doesn't work, then try using SEROUT2 PORTC.6,84,["Hello World",13,10].

If you get nothing on screen, I would check my connection from RC6 to the PC side of the MAX IC, serial cable, etc.

If you have an O-Scope set it in a continuous HSEROUT loop while watching the MAX TX out pin to PC.