Onslaught verion 1.0.4 requires a new "Scenario Data" file in order to boot and run.
Installing the new Scenario Data
Locate the old "Scenario Data" file inside the "Game Data" folder, inside Onslaught's "Data Folder".
Along with the "Scenario Data" file you will see the "Sound Data" and "City Name Data" files.
Place the "Scenario Data" file ONLY into the trash.
Place the new "Scenario Data" file into the "Game Data" folder.
Make sure your Onslaught applications are version 1.0.4.
Play away!
If Onslaught doesn't boot...
You either did not replace the "Scenario Data" file with the new version, or you have not updated your Onslaught applications to version 1.0.4. If this happens to you, review the update instructions and try and figure out where you went wrong.