According to TechNet, the SideLoad Requirements are:
Windows Runtime-based apps must be cryptographically signed and can only be installed on a computer that trusts the signing certificateYou must activate a sideloading product key or join the PC to an Active Directory domain that has the Allow all trusted applications to install Group Policy setting enabled.Until the PC meets all of the sideloading requirements, app tiles on the Start menu will show an “X” in the bottom-right corner to indicate that a problem is preventing the app from running.
Microsoft is serious about security, so no Malware can get through from the Windows Store or from a non-store application getting infected through sideloading. NOTE: Microsoft has now made it easy to SideLoad apps on Windows 10 PC.
SideLoad Non-Store Apps to Windows 8.1
Here is how to do it: Step 1: We need to enable the Group Policy that allows us to sideload non-store applications.
Hover the mouse to Left end corner and right-click on Start
Click on Run and type in gpedit.msc to open Group Policy Editor.
Go to Computer Configuration – > Administrative Template – > Windows Components – > App Package DeploymentDouble-click on “Allow all trusted apps to install”.
Click on “Enabled” then click OK
Now you are ready to install non– store applications on Windows 8. Step 2: Now we’ll install the application using PowerShell command.
Go to your Windows 8 UI and start typing in PowerShell.Right-click on the PowerShell and click on “Run as administrator”.
That will open the PowerShell in administrator mode. It should say Administrator : Windows PowerShell on the top.
Now type in add-appxpackage C:\app1.appx –DependencyPath C:\winjs.appx
You are all set! Now you should have the application installed. This method is for IT professionals for deployment on a large scale. I personally couldn’t test this method since I was not able to find a non-store application.
