If you read Borland's documentation on how to debug components, you'll notice that there is no documentation on how to debug a component editor.

Here's my take on how to debug a component editor.

In an instance of Delphi IDE, load the design time package containing the component editor. Ensure that this design time package registers the component editor, and a component that requires the editor.

For my case, there's a Register procedure that reads

  RegisterComponentEditor(TMyComponent, TMyComponentEditor);

Then, compile and install the package. Go to the Run menu in the IDE, choose the Parameters menu item, and set host application to the Delphi IDE, eg, C:\Program Files\Borland\DelphiX\bin\delphi32.exe. Then, click Load. After a few seconds, the CPU window appears, press F9, or click on the green play button on the Debug toolbar.

After the second copy of Delphi loads, create a new application in it, and drop the component in the form. Then, activate the component editor.