Here is a picture showing how to create a sample set, and where to put it:

These are the available keywords in the definition.txt file:

• %notename : C1, C2, C3, D#3, F#4, etc.
• %midinote : 36, 48, 60, 63, 78, etc.
• %velocity : 1, 43, 72, 127 (maximum), etc.