See if this trick here helps you: UE4 install failed with code MD-0005 - Getting Started & Setup - Epic Developer Community Forums
If that does not work try contacting Epic games support via email here: http://help.epicgames.com/ (The Email button is off to the right of the page, I would suggest perusing this list for future help. And don’t forget the answerhub for other problems regarding UE4! :D)