Rob, I had the same problem. None of the plugins I tried would let me open cr2 files in pdn. All pdn gave me was the same error message: "There was an unspecified error while opening this file".
I deleted and reinstalled all of the plugins (RAWFile, RawLoaderPlugin and RawReader) and even paint.net itself. After hours of this I was nearly ready to give up and delete pdn from both computer and mind.
However, I found a sollution. I suggest you try the <link to the illegal plugin pack which redistributes plugins without permission removed by EER. Do NOT post this link again & use it at your peril - you have been warned.>
This worked for me. Hopefully this might be helpful for you and others.
Edit: Don't use the plugin pack. Search for RAWFile.dll ver. 188.8.131.52 (or later, probably).