App Inventor Android Emulator for Mac

Hi all, I am new to App Inventor. When I tried to download the emulator Version 3.0c in following website: http://appinventor.mit.edu/explore/ai2/mac

It mentioned "MITAppInventorSetup.pkg cannot be opened because it is from an unidentified developer." macOS cannot verify that this app's free from malware.

What should I do? Thanks.

If you Control+Click (or right click) on the file and select "Open" from the menu, it should launch the installer even though it is not signed.