When you are choosing a scenario to play, you can opt to create your own custom game. This brings up a dialog that lets you enter the scenario file for it. This is a text file in the Scenario File Format.
To modify an existing scenario, select it before you click on the Custom Scenario button. The Scenario editor will load then with the scenario file for the selected sceanrio. You can then edit it and modify what you want to change.
To use someone else's scenario, first call up the scenario editor. Then cut and paste the text for the scenario from their web page, e-mail, or text file.
To create your own scenario, it is strongly suggested you edit it in a text file on your system. To test it, bring up the sceanrio editor and cut and paste the text into it.
If you highlight a line containing a ship definition, you can click on the "Ship Designer" button. This will bring up the Ship Design Dialog for simplicity of editing.
When you are done with the scenario you can click OK to close the editor. The sceanario will now appear in the Choose Game panel. Be sure to select it before continuing!