opto-isolator 6N137 output not toggling
You should hew to the datasheet recommended minimum voltage of 4.5VDC if you want reliable operation, however it does not appear that is the proximate issue. 

Your measurement of +5V at 2 and 0V at 3 indicates you have a wiring problem or you've damaged the LED (perhaps by accidentally connecting it without a resistor for even an instant). The LED forward voltage should be around 1.3-1.6V- and that is a good indication that it is actually emitting light. If it is much less or much more, that indicates a short somewhere (perhaps internally) or an open (again, perhaps internally). In your case it appears to be open. 

As an aside, your 200 ohm resistor is a bit on the low side- I suggest keeping the nominal current more like 10mA. That is not what damaged the LED (if that's the problem) just better for long-term reliability.