The following error:
ERROR: [1237-7-5-00000000] User list file 'C:\Program Files\Wingra\GroupWise Migrator for Exchange\UsersToMigrate.csv' has no 'DisplayName' column."
This error indicates that the CSV file which is being used is not properly formatted and is missing data which is required for the migration to work properly.
When modifing the UserstoMigrate.csv or the UserstoMerge.csv files do not remove columns or this error may occur.
Take a copy of the original UserstoMigrate.csv file which contains the Display Name column in it and use it for creating your CSV file(s) to be used with the Groupwise Migrator for Exchange. Make sure that you do not remove columns or the top row, remove just the rows which are objects from the source.