I need MP Navigator EX 2.3 but when I click on the icon to install it, the window says it can't be opened because it's from an unidentified developer and not from the Mac App Store. How can I work around this?

Hold the control key or right click and Open, confirm and it won't bother you again on that program.

Clearly a window appearing with the notice and a option to continue (like Windows does) would have been preferred, but Apple is trying to steer everyone into getting their programs only from AppStore. 😟

The easiest way is to right-click (or control-click) on the installer and select "open". You'll be told that this is from an unidentified developer but given the opportunity to install it anyway.

Regards.

Open the Security & Privacy preferences. Change the option for "Allow applications downloaded from:" to Anywhere.

Making that change permanent is not recommended. You forfeit some important protection if you do. Better to just use the right-click option DS_Store and I gave above for downloads you are certain about.

"Program can't opened" issues