Megalith Card dropped by Megalith (100.00%) provides +7 MDEF if the refinement level of the footgear is +5 or less. It the footgear is refined to more than +5, then the card will not work.
What players can do is that they can refine their footgear at the Refine Master using a Refine Deed, such as a +7 Armor Refine Deed (dropped by Hydro in Eden Dungeon). After that, they can de-refine the footgear by 2 reductions at the De-Refiner for a price of 1 Mithril Coin. The footgear will then be refined to +5, and the card will work.