The wording of the Swordsage maneuver Death Mark is as follows:
When you use the strike, you channel overwhelming fiery energy into the body of your foe. In addition to dealing normal damage with your attack, you cause fire to erupt from your enemy's body in a spread.
Tome of Battle p.52
There is no mention that the melee attack has to actually hit to cause the fire damage - does that mean the enemy bursts into flame, taking 6d6 extra fire damage in any case?
The first clue lies in the description you provided- specifically, the words "in addition to." That would imply pretty strongly that you deal the fire damage upon dealing normal damage, which does not occur when you miss.
But we can actually find a much more precise source. Note that Death Mark has a range of "melee attack." On page 45 of Tome of Battle, you'll find the descriptions of each range. Here's the one for melee attacks:
Melee Attack: The maneuver affects any creature you make a successful melee attack against.
You unequivocally have to hit your enemy before the extra fire damage can take affect. On the bright side, the fire damage from Death Mark also hits everyone else nearby:
All creatures in the area, including your enemy, take 6d6 points of fire damage...
...And in a case of specific beating general, you won't need to make a successful attack against them too.
As long as you land that first blow.
