automating plug-in parameters in logic?

Just wondering if anyone can help me out with this...

i want to be able to change a synth or plugin effects parameters in MIDI after its been recorded. Also, i use a USB keyboard with assigned knobs, which i can assign the parameter i want to affect. But, i'm not good enough to play and change it at the same time. Is there a way to used the assigned knob to change the parameter 'live', whilst i listen to the track?

Thanks for reading