@hbilhan No problem! The servo headers are located to the left of J1 - J5, and are labelled according to their corresponding Motor assignment (M1 - M6) (pictured below).
In each case the top pin in each group of three is the servo signal, the middle pin is 5V, and the bottom pin is GND. Since you have a 4-way motor shield and you are using M1 - M4, you can't really use those servo positions. I mean you can, but the behavior is just going to mirror what's going on with the corresponding DC motor. But M5 and M6 are certainly available to you.
From the User's Guide on page 5:
- Speed is controlled in the normal way by PWM using the analogWrite function
- M1 on DIG8 or DIG11, M2 on DIG9, M3 on DIG5, M4 on DIG10, M5 on DIG6, M6 on DIG3 or DIG4
So DIG6 runs the M5 speed (pwm) signal, and M6 in the jumper default configuration run is run by DIG3 (which we know is kind of a problem in your application). You can move J2 to the alternate position and the M6 will be run by DIG4, but on an Uno, DIG4 doesn't have PWM capability (whereas it does on a WildFire). You might be able to work some software-based PWM on it but Servos have pretty tight timing tolerances (i.e. generating 1-2ms pulses with something like 3.9us steps in between) so that might prove challenging.
Running a DC motor is quite different from running a Servo from a signaling standpoint. And you have to keep in mind that Timers couple pins behavior to one another when it comes to PWM.
- Timer2 kind of binds DIG3 and DIG11
- Timer1 kind of binds DIG9 and DIG10
- Timer0 kind of binds DIG5 and DIG6
So because of that running a DC motor on M3 and a Servo motor on M5 could be tricky. In truth I haven't tried it so it might "just work" but I suspect there may be issues.
Hope this helps!