How to add C-wire from Laars mini-term
With only two wires between the thermostat and the boiler, you're not going to be able to power the thermostat, and call for heat.  You'll need at least three wires for that. 

The C terminal on the controller is likely the COM terminal, though I can't say for sure. According to the schematic, pin 2 of the PCB connector is C. But I'm not sure where it's routed from there on the board.