Added Mox Diamond (STH). Tested and working.

The 3rd auto-line is necessary because without it Mox Diamond will give you the chance to add mana without discarding a land when the "action-choice-window" opens.

projects/mtg/bin/Res/sets/STH/_cards.dat

@@ -366,6 +366,17 @@ toughness=3
 abilities=cantattack,cantblock
 [/card]
 [card]
+text=If Mox Diamond would enter the battlefield, you may discard a land card instead. If you do, put Mox Diamond onto the battlefield. If you don't, put it into its owner's graveyard. {T}: Add one mana of any color to your mana pool.
+id=5193
+name=Mox Diamond
+auto=moveTo(mygraveyard) notatarget(land|myhand)
+auto=moveTo(mygraveyard) notatarget(this|myBattlefield)
+auto=@movedTo(land|mygraveyard) from(myhand):lord(mox diamond) {T}:Add{W} && lord(mox diamond) {T}:Add{U} && lord(mox diamond) {T}:Add{B} && lord(mox diamond) {T}:Add{R} && lord(mox diamond) {T}:Add{G}
+mana={0}
+rarity=R
+type=Artifact
+[/card]
+[card]
 text=Sacrifice Morgue Thrull: Put the top three cards of your library into your graveyard.
 id=5191
 name=Morgue Thrull

projects/mtg/bin/Res/sets/STH/todo.dat

@@ -374,14 +374,6 @@ text=Whenever a creature is put into your graveyard from the battlefield, put th
 rarity=R
 [/card]
 [card]
-id=5193
-name=Mox Diamond
-mana={0}
-type=Artifact
-text=If Mox Diamond would enter the battlefield, you may discard a land card instead. If you do, put Mox Diamond onto the battlefield. If you don't, put it into its owner's graveyard. {T}: Add one mana of any color to your mana pool.
-rarity=R
-[/card]
-[card]
 id=5194
 name=Mulch
 mana={1}{G}