To side-load windows apps on a device with using a windows side-loading key follow these instructions
First of course you need a windows side-loading product key. The only way to purchase this key is via the Microsoft Volume Licensing program.
Install side-loading key on device
First, open an elevated command prompt.
execute the following to install the side=loading product key
slmgr /ipk [SIDE-LOADING PRODUCT KEY]
then execute the following command to activate the key. An internet connection is required to do this.
slmgr /ato ec67814b-30e6-4a50-bf7b-d55daf729d1e
Allow all trusted apps to install
On a Windows 8 Pro Device do the following
Execute the local Group Policy Editor (gpedit.msc)
Navigate to Local Computer Policy->Computer Configuration->Administrative Templates->Windows Components->App Package Deployment
Set the Allow all trusted apps to install option to Enabled
this should create the following registry entry
and it should be set to a DWORD value of 1
App Signed with a Trusted Certificate
The app to be installed has to be signed with a trusted code signing certificate