"App can't be opened because it is from an unidentified developer"
...then go to system preferences >security and privacy - under allow applications downloaded from - click on
To run an app or dashboard widget blocked by Gatekeeper without compromising security, right click the app/widget and select Open. You will be prompted once to confirm you want to run the app and it will then be marked safe by Gatekeeper. Reserve this for apps/widgets that you trust. Remember that the new feature is there for security and it is okay to bypass it when you have a trustable app/widget but not for any and all apps/widgets that you download.