David,
As Al_S sais, it's just a matter of setting a callback in the double click, however if you are reading the cell value where the double click took place then you need to be more careful in the way you read the cell value.
When you get the callback the value of the selected cell is not updated yet, so you need to use PostDeferredCallback and read the values in that second call. There is some information here:
http://exchange.ni.com/servlet/ProcessRequest?RHIVEID=101&RNAME=ViewQuestion&HOID=50650000000800000023280000&ECategory=LabWindows%2FCVI
I hope this helps.
Regards,
Juan Carlos
N.I.