The pin is PCI28/OC1B/(Digital 12)/PD4/pin18 on ATMEL ATMEGA644P/SANGUINO.
So: pin=12.
I checked wiring.h and wiring_analog.c for sanguino and everything is OK. Translates to:
sbi(TCCR1A, COM1B1); // connect pwm to pin on timer 1, channel B
OCR1B = val; // set pwm duty
PWM works great at >0.