Logic Pro 8, 9, X make it very easy to use your MIDI controller's wheels and knobs to control mixer and plug-in parameters. Here's how to assign knobs to parameters:

1 • Press Command+L* 
(L as in Learn) - The Controller Assignment window opens. Move that window out of the way: leave it open, but don't even worry about what's in that window.

2 • In Logic, click the fader or knob you want to control. 

3 • On your MIDI controller, move the knob you want to assign to that control. 
You can repeat steps 2 and 3 for as many knobs as you want.

4 • Close the Controller Assignment window 

You're done. 

* If Command+L is not assigned you can assign it (or any other key combination) yourself in your key command window (Logic Pro > Preferences > Key Commands): search for the key command "Learn new Controller Assignment".