I don't believe that it takes a full path. I believe that you must only enter the filename, i.e., 9743218.ies.
First, make sure you copy the .ies file to the location specified for .ies files in the C:\Program Files\Autodesk Revit Systems\Program\Revit.ini file... by default this is:
IESFileLocation=C:\Documents and Settings\All Users\Application Data\Autodesk\Revit Systems\IES
HTH,
Martin Schmid
Autodesk Consulting
Martin Schmid
Product Line Manager
Mechanical Detailing and Electrical Design
Architecture, Engineering, and Construction
Autodesk, Inc.