Choosing A Virtual Instrument
Assuming the track is a Virtual Instrument Track, click the instrument piano keyboard button to bring up the Instrument Preset window.

This will bring up the instrument preset window.

Click on a category and then click on a preset.

Click the [x] button on the top right of the window to hide the instrument dialog.  Alternatively, you may click the instrument button on the track header to toggle the visible state of the instrument preset window.

There are three special categories: <Custom>, <Instruments>, & <External Devices>.

This means that you have made your own custom virtual instrument.  
Learn more about creating a custom virtual instrument patch.

This category will show all available virtual instruments on your computer, including any external ones.  The bundled synthesizers include Acoustica Instruments, Impulse, MinimogueVA and VB3 Organ.  VSTis are the only supported type of plug-in. If you have a virtual synthesizer that is not showing up, you may have to edit your VST Effect folders.

Acoustica Instruments - This is a general MIDI synthesizer with additional banks of sampled sounds.
Impulse - This is a polyphonic subtractive analog synthesizer.
MinimogueVA - This is an emulation of the popular MiniMoog synthesizer.
VB3 Organ - This is an emulation of the popular Hammond B3organ.

When you click on a virtual instrument, the window will expand and you will see the following:

Click on the preset column to select a preset.  
Learn more about creating your own virtual instruments.

<External Devices>
This category will show all external MIDI outputs or sound card MIDI synthesizers.  If you have an external synthesizer connected via a MIDI interface, select the appropriate MIDI output interface.  Please note that you will not be able to render or burn the audio made by these synthesizers unless you record them to an audio track first!

MiniMoog™ is a trademark of Moog Music, Inc.
Hammond B3™ is a trademark of Hammond-Suzuki, Inc