Double-click on the audio region to open the sample editor. In the local menu choose Factory>Audio to Score, where you'll then be prompted to choose some parameters. There are some useful presets there, e.g. "Drums Fast." Click OK and a MIDI region will be created. Sometimes the new MIDI region is invisible because it lies directly beneath your original audio region. Just drag the audio region out of the way temporarily to access the MIDI region and just drag that down to an instrument track. Then go nuts!