The Expression Pedal must be assigned somewhere, otherwise it won’t have any effect. Go to the Mod Matrix and set a modulation with Pedal as a source and assign it to any destination you like.
If you want to control the volume with the pedal, set the pedal to zero. Create a modulation with Constant->Volume and adjust the amount into the negative area until the output level of Quantum is completely silent. Attention: Negative values for volume will also produce audible sound! You will have to hit the zero level exactly. When done, push the pedal to 100%. Set another modulation with Pedal->Volume. Slowly increase the amount until you can hear something. When the pedal is fully engaged, you can control the maximum loudness with the amount. Keep in mind while using this method, nothing will be audible when the pedal is not plugged in.
Or just use one modulation Pedal->Volume with a negative amount, then the volume gets softer as you press the pedal. In this way something will be audible as well when there is no pedal plugged in.
IMPORTANT: Remember to save the sound after the modulation has been set. The modulations are stored with the sound and have to be done individually for every sound.