Hi,
What LabVIEW version you use? If you use 6.1, there is Event Structure, that very easy for using. There are examples in LV help.
If you use previous versions of LV, you must check value of control, if it changed.
Another approach is to put ActiveX numcontrol, for example CWNumEdit of NI and use ActiveX event (valuechanged)
Look attached examplewith 2 approaches.
Hope that helps.
Nadav Chernin
nchernin@chiaro.com