There are various ways to hear an Instrument:

• Click notes on Kontakt’s on-screen keyboard. However, they will all be the same velocity, about 120. (Click the Keyboard icon in the Main Control Panel at the top of the rack to show or hide the On-Screen Keyboard panel.)
• Use a MIDI keyboard with Kontakt in standalone mode. To play the instrument from your physical MIDI keyboard, set your MIDI keyboard to transmit on the same channel as the Instrument in the Kontakt rack.
• Play Kontakt Instruments from your sequencer. You can use Kontakt in plugin or standalone mode.
• Plugin: In your sequence, add an Instrument track and map a MIDI track to play the Instrument track. Then, on the MIDI track, record notes or set up a loop playing the note range you want to hear.
• Standalone: For your Sequencer MIDI track, select a MIDI output and channel and in Kontakt select the same MIDI output and channel in the Instrument header. The MIDI Ch field is under the Instrument name. 

  Note: In Digital Performer, you can set up Interapplication MIDI in your sequencer to make outputs available to Kontakt in standalone mode (see Elusive Music: Using Kontakt Standalone with Digital Performer)

To hear keyswitched Groups
If your instrument uses keyswitches to enable different sounds, you’ll see the keyswitch notes in red on Kontakt’s On-Screen Keyboard panel.

To hear different Groups that are controlled by keyswitches:

• Click a red note on-screen, OR
• Play the equivalent note on your MIDI keyboard. Most instrument use a very low octave for keyswitches. For a 60-key keyboard, you’ll have to transpose down an octave to bring those keys into range.

Demoing samples
The Audition button at the bottom of the Files panel only seems to play samples, not instrument definitions. So it won’t help you select an Instrument. Use it when you want to find samples to add to an instrument.