Marginal efficiency of capital - i've never heard that phrase before, but i'd imagine it would be that as capital accumulates and all other resources remain fixed, the efficiency of capital decreases. i.e diminishing marginal returns to capital...?
for a example if you have 3 chefs in a kitchen with one oven and u add an oven - each chef can make 10 more cakes than before. If you add another oven, because the kitchen is less spacious, each chef may now only be able to make an additional 8 cakes - still more cakes than before, but the use of capital is less efficient?
*sigh* sorry, best i could come up with...