You can sideload LOB Windows Store apps apps on devices running Windows RT and on devices running Windows Server 2012, Windows 8 Enterprise, or Windows® 8 Pro. You can't sideload LOB Windows Store apps on any other editions of Windows in your enterprise. However, you can use a developer license to add apps that are in development to other editions of Windows. For more information about testing apps that are in development, see Get a Developer License.
Before you can add sideloaded LOB apps to a Windows image:
Before you can run sideloaded LOB apps:
Until the device 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.
Open the Group Policy editor (gpedit.msc). For example, on a computer that is running Windows 8, from the Start screen, type Edit Group Policy, click Settings, and then select the Edit Group Policy setting from the search results.
Under Local Computer Policy in the left pane, click to expand Computer Configuration, Administrative Templates, Windows Components, and then App Package Deployment.
Double-click the Allow all trusted apps to install setting.
In the Allow all trusted apps to install window, click Enabled and then click OK.
Setting the Group Policy to allow trusted applications updates the following registry setting: HKEY_LOCAL_MACHINE\Software\Policies\Microsoft\Windows\Appx\AllowAllTrustedApps = 1
The entire TechNet Article can be read at - http://technet.microsoft.com/en-us/library/hh852635.aspx#SideloadingRequirements