The .accdr file extension is used to make the database file read-only and prevent changes to it. As you have discovered, changing it to .accdb lets you edit the file, but it also leaves you open to file corruption and other errors. I would change it back to .accdr which will again make it read-only. However, in the long run you should have the developer incorporate your needs into the application - otherwise when they do put out a new version, you will have to make the same changes again.
You can't see the view if you don't climb the mountain!