This is no longer required. Starting from version 2.2.x ORM Designer has a valid Apple developer certificate.
"OrmDesigner2" can't be opened because it is from an unidentified developer.
We know about this issue. It's caused because currently we don't sign compiled executable with Apple Developer key. We have this as one of our priority task, but it takes time to gain all requirements from Apple.
In the meanwhile if you want to use ORM Designer also on Mac OS Mountain Lion 10.8, please allow executable of unsigned applications on your machine. To do it, please follow these steps:
Open System preferences and choose Security & Privacy
Click on the Lock icon and enter your credentials:
Click on "Anywhere" radio button and confirm your choice with clicking on "Allow From Anywhere" button.
Now you will be able to execute ORM Designer without any problems.