Side-loading apps requires generating a Windows 8 side-loading key. This can be done using PowerShell.
- Open the start screen
- Type in
powershell
- Right-click on Windows PowerShell
- Click
Run as administrator
Once PowerShell opens, type in the following and hit enter:
Show-WindowsDeveloperLicenseRegistration
(Alternatively, you can type Show-Wi and then hit Tab to auto-complete.)

This will open a dialog asking if you agree to the Windows 8 Developer License:

Agree to the license (if you choose to do so), and then you will be prompted to log in to a Microsoft account:

Once your account is verified, you will see the following message saying that you now have a developer license, and stating the expiration date. You will have to run this process again after the expiration date.

You can now close PowerShell. If you want to check your developer license status later, you can use the following PowerShell command:
Get-WindowsDeveloperLicense
It will return an object stating whether the license is valid, and what the expiration date is. You can also remove/deactivate the developer license using the following PowerShell command:
Unregister-WindowsDeveloperLicense
For more documentation and details, see this TechNet article.