In that case, the load resistor in my diagram becomes a pull-up resistor and the connection to your ballast input comes off of the circuit between the pull-up resistor and transistor.
This will invert the PWM signal, so you'll have to account for that in software. When the PWM signal is high, the transistor will conduct to ground and put a low on the ballast input. When the PWM is low, the transistor will be off and the pull-up resistor will put a high on the ballast input.
The values of the resistors can be higher in this case too. You could probably use 10k resistors for both, depending on how the ballast reacts. You may need to test with different values to find the best setup.
EDIT: I modified and reuploaded the diagram, which I realized is not the best thing to do right after doing it. I should have put up a new image instead. Oh well...