I'm not familiar with that module, so I'm talking out of my backside on this one, but:
1. Are there configuration jumpers on the module?
2. Are there configuration bits in the I/O table of your program? The analog module for the SLC platform (1746-NI4???) needs to be configured by setting I/O bits.
3. Is the module TRULY 4-20ma or does it read a slightly wider range? I had a scaling issue once because the module was really scaled 0-21ma.
Even if you've missed one of these, I can't really imagine why you'd be seeing a negative value (unless your transmitter isn't transmitting, or the module isn't receiving). But, sometimes strange things DO happen.
If all else fails... jiggle the handle.
AK