The new application "iNotepad", although it may appear similar to the old application "Notepad 9.0", in reality is different under the hood. The interface of the app has not changed that much because people loved it but our team of engineers has rewritten this app in the foundations to make it more modern, fast and able to take advantage of new technologies. This also required the adoption of a new document format.
The application uses documents (like many other Mac applications, such as Word and Pages). To find them go to the Finder and browse the folders you usually use, such as the "Documents" folder.
To import the document files of Notepad in iNotepad (with a "i" in the name) download Notepad File Converter and follow the instructions you'll find in the main window of the app.