I havn't got that computer..
Hwvr, I think the sensor either works or it doesn't - i.e I am not sure where the sensor is placed will effect the distance - it will either be zero or what it is reading. Still worth checking again that :
1. The sensor is pretty high up the fork, if it is too far fm the computer, it may not be in range.
2. The magnet passes very close to the sensor - it shud pass accross the middle of the senser and be less than 2 cm - this can vary with system, I normally have pass about 5mm.
When you spin the tire, do you get an accurate speed reading ? also if you get close you shud hear a quiet click every time the magnet goes past - if this all seems OK, you may need to review your distance input again - can you not ask Trek for some advice on this ?