You need to define the constant PRECISION and assign it a value equal to the number of bytes to use for the values in the math routine. And you need to do this before including N-Bit. In this case it looks like Richard opted for 32bit values (4 bytes)
Code:
PRECISION CON 4 SYSTEM    '32bit values
Include "N-Bit_Math.pbp"
If you open the N-Bit file you'll find a link to the thread on this forum: http://www.picbasic.co.uk/forum/showthread.php?t=12433

/Henrik.