I had this problem too as the normal keyboard combinations for reproducing the copyright symbol don't work in PDN as they are assigned to other functions.
If you go to Copyright Authority website they have a selection of images that you can copy, free of charge, and use in your images.
Just copy the image from the webpage and paste it into PdN, you can then manipulate it how you would like (resize, recolour, etc...) and save it for future use.