With WindowsRT Microsoft has made a bit of an odd platform that only accepts apps from the Windows store and does not provide a simple way to install any custom built apps. The easiest way to sideload an app onto a WindowsRT tablet is to set it up as a developer test device. Unfortunately this does require you to be able to log-in to a Microsoft developer account. Not only is this account not free, but you will be required to reconfirm your credentials every three months. Still this is far easier then the other options to sideload onto WindowsRT.
First you need to find the Windows PowerShell app open it using the “Run as administrator” option. Then confirm yes at the UAC prompt. Open the keyboard to type
Show-WindowsDeveloperLicenseRegistration and hit enter. Then agree to the prompt, and then log-in to your developer account.
In Visual Studio create a Windows Store app in the same manner you would as if you were planning on releasing the app to the store. However once the executable is generated in the AppPackages folder, instead of uploading the .appxupload file find the similarly named test folder and transfer this whole folder onto your WindowsRT device. For WindowsRT devices without support for USB drives this is a challenge all on its own, where the best solution may be to email yourself a .zip file.
Now with the folder on your WindowsRT tablet find the Add-AppDevPackage.ps1 file, touch and hold to bring up the right-click menu and select “Run with PowerShell”. Confirm that you do in fact want to open the file, and then follow the multiple prompts within PowerShell. Eventually the app will load be able to run the app as if you had installed it from the app store.