If you are using a Garmin GPS unit for this work, then use DNR Garmin to connect to the GPS unit and directly create a KML file. DNR Garmin is free.
If not, save your excel file to a comma delimited *.csv file, and use a tool like this to convert it to KML.
http://freegeographytools.com/2007/importing-spreadsheet-data-into-google-earthIt appears that your data is in Latitude (Northing) and Longitude(easting), but just to be sure, you need to have your coordinates in Latitude and Longitude for it to plot correctly in Google Earth.
Cheers,
David