When you have a Software Instrument track selected in GarageBand 2, the Instrument Tuner button (the pitchfork) in the Transport area will not light, and the menu selection (Control > Show Instrument Tuner) will be dimmed.
The Instrument Tuner is only available for Real Instrument tracks. To display the tuner, select a Real Instrument track; you will then be able to open the tuner by either clicking the Instrument Tuner button or choosing Show Instrument Tuner from the Control menu.