creat a sequencer track connected to that effect. make sure this track is enabled to record (the little red light shows up next to the track) now press record. on the actual effect in the rack view manualy adjust your parameters with the mouse and/or midi controller. you can also edit automation by first creating the track for the effect as stated before and then right click on the parameter you wish to edit or automate and then select "edit automation". you can now adjust the effect in the sequencer window.
you can do this with almost every knob or parameter in reason (freq, volume, pan, feedback,effect on and off) just remember to create a track specifically for that machine , synth, or effect. now go tweak!!!!!!