The easiest way to get your layouts to open in MIDI Designer is to have them in Mail, Dropbox or Google Drive. Then you can use "Open in..." to open them in MDP2.
From your iPad or iPhone, just touch and hold (long press) on the link. This should allow you to specify to open in MIDI Designer.
READ THE MANUAL ENTRY ON THIS TOPIC under "Opening Layouts in MD"
You can also use iTunes File Sharing to transfer files.