The Kernel Image file should be found in the WPILib folder in your WindRiver installation directory ("C:\WindRiver\WPILib" by default). Make sure you use the right file for your cRIO version; "cRIOFRC_vxWorks" for an 8-slot cRIO or "cRIOFRCII_vxWorks" for the newer 4-slot version.
If the WPILib directory is not there, make sure you have installed the C++ update for 2013 found here:
http://firstforge.wpi.edu/sf/frs/do/...tware_up date .
There is an excellent set of instructions for the whole process found over on the control system documentation site:
http://wpilib.screenstepslive.com/s/...iver-workbench .