Nice job on the schematic! Wasn't hard, was it?
I''ve renamed the files with _s to make that easier.
I think I would have put LED D1 right below R1 to the opto collector, have the collecter going to D3, and have the opto emitter go right to ground. That's the way an NPN transistor would normally be wired.
When the opto is off, your internal pullup and the R1/D1 combo keep it high. When the opto turns on it should pull it down to ~0.7V or so.
Ah - I see it didn't work without the LED either.
The pulse needs to be some minimum width for the interrupt to work - do you have any way to put a scope on it? If you can see it flash, that seems like the width would be enough.
Instead of falling, can you try LOW level for interrupt?