What this error box shows is the Gatekeeper functionality that notes if an application has not been signed by a developer registered with Apple.
It prevents people from accidentally running apps that are unsigned.
You can make this and only this application run by bypassing this gatekeeper check by opening a Finder window showing the file and right click on the file and select open, you will then get a dialog box similar to the one you have which will also have an OK button allowing you to open the application.
You can also relax the settings in System Preferences to stop this check happening and allow any application to run (unlock and select the button named Anywhere and run all the unsigned apps you want to trust and then revert the settings to more secure)
However from other answers this might only get you a bit further as this does not deal with Rosetta/PowerPC issues.