Memory Map have recently changed their free app so that you can transfer maps/charts from your desktop, through iTunes to the free app on your iThing & removed the paid app.
Apple have realised that there is no money in this for them & have pulled the MM app from the app store. MM have re-submitted a changed app but I don't know what the new terms & costs might be.
You can still plan flights using MM and export the routes as a gpx file (right click>save as) & import into Air Nav Pro.