Here's a video showing how this functionality is intended to work:
This should work with the phaser algorithm as well, but in the phaser algorithm (and in others as well) some parameters have greyed out light rings to indicate that they cannot be mapped to the expression pedal. This can be for a variety of reasons, but most commonly if a preset is set up to use the expression pedal as a modulation source, it cannot also be used to change the parameters.
