How do I install the HiDPI extension to enable retina?

Mac-to-iPad Mode only

In Security & Privacy, if you are blocked from clicking "Allow" for the HiDPI extension, this is most likely a MacOS/Gatekeeper issue. There are many possible reasons why this won't allow a click, but here are some things to check for:

Third-party applications that emulate mouse or trackpad functions are in-use
Utilities or applications like MagicPrefs, Synergy, BetterTouchTool, HyperDock, etc., can be blocked from clicking "Allow." Try disabling them and see if there is any difference. 

Use Safe Mode
If the hardware mouse didn't work, try is restarting the Mac in Safe Mode, and access Security & Privacy and see if it clicks from this Mode.

Safe Mode would disable any third-party plugins.
Afterward, restart as normal.

How to restart in Safe Mode:
-Restart your Mac. 
-Immediately after you hear the startup sound, press and hold the Shift key.
-Release the Shift key when you see the Apple logo appear on the screen.
To leave safe mode, restart your Mac without pressing any keys during startup.
More on Safe Mode can be found here: https://support.apple.com/en-us/HT201262

Other methods to try

• Close Luna Mac app and try to click "Allow
• Turn off Wi-Fi and click "Allow
• Try a hardware mouse and click "Allow"