Once you have downloaded the installer .pkg file from the NodeEra website you will need to run the installer.
Locate the .pkg file in your downloads folder.
Right click and select open with Installer App.
You will see the message above. The NodeEra installer .pkg file is signed with an Apple supplied certificate but since it hasn't been approved for the Apple store you might get this message.
Go To the System Preferences Application (see below).
Select the Security and Privacy app (circled above).
The Security & Privacy App will give you a chance to open the installer (circled above). Click on the "Open Anyway" button.
Now you will be able to run the installer by clicking on the Open button (circled above).
Once the installer starts just following the directions on screen.