To debug a Plug-In DLL in Delphi 7, go to the Run > Parameters menu item in the IDE, and select the PL/SQL Developer executable as the host application. When you now run your project, PL/SQL Developer will be launched allowing you to set breakpoints and step through your DLL code.
Note that it may be useful to set the PL/SQL Developer Plug-In directory to your Delphi project directory.