When using LoadBuilder, there are no available fields available when selecting the Usage Field.
The Usage Field property is expected to be a numerical formal. Though numbers may be listed in shapefile, check the format for the column to make sure that it is not set as a Text field. If it is, convert this to a numerical field. You should then see this column as being available in the Usage Field pulldown menu
Troubleshooting and Understanding LoadBuilder
LoadBuilder importing zero flow loads