You can edit controller information in the Midi Draw Area, see this Help page for GarageBand 10:
http://help.apple.com/garageband/mac/10.0/#gbnd0bf9dcc3
In addition to editing the notes in the Piano Roll Editor, you can view and edit controller information using MIDI Draw. Many music keyboards include controls for pitch bend and modulation. The pitch bend and modulation controls are often wheels, placed to the left of the lowest keys on the keyboard. Some keyboards also let you connect a sustain pedal or foot controller.
As you play, you can move these controls to add expression and character to your music. You can view and edit the movements of these and other MIDI controllers in the Piano Roll Editor. MIDI Draw can show controller information for Volume, Pan, Pitchbend, Modulation, Foot Control, Expression, Sustain, or Pitch Bend.
The MIDI Draw area below the Piano Roll Editor note display shows the (data, value) for each note in the MIDI region. A control point at the left edge (beginning) of each note shows the value for the chosen controller type.
See the page for details and screenshots: http://help.apple.com/garageband/mac/10.0/#gbnd0bf9dcc3