After I download software from dotphotobooks.com, I get an error message that says:

dotphotobook can’t be opened because it is from an unidentified developer.

Your security preferences allow installation of only apps from the Mac App Store and identified developers.

This is a security preference on your Macintosh that prevents apps that are not obtained through the iTunes App store.

1. Go to your Security & Privacy settings.
   
2. Unlock them.
   
3. Choose "Allow apps downloaded from Anywhere"

Please refer to this screenshot:

You may also receive an error message relating to certificates, which you can ignore to run the software.