The Custom Datums are appended to the end of the Didger.ini file. You can find them at the bottom, under the [Custom Datums] header. Each custom datum has its own line.
To locate the Didger.ini file:
Didger 3: The Didger.ini file is located in the Didger installation directory (by default this is C:\Program Files\Golden Software\Didger3).
Didger 4: Please see Where is the Didger.ini file for Didger 4? It is no longer in the installation directory.
Didger 5: The Didger.ini file is located in the Didger AppData directory (by default this is C:\Users\<username>\AppData\Roaming\Golden Software\Didger 5).
Updated October 20, 2016