There is a fan created Heroes VI manual that contains all the gritty details of the damage calculation. This is not an official manual, so no guarantee it is correct.
Essentially there are two calculations:
Raw Damage = Creature Damage * Stack Size * Attack Modifier
Creature Damage and Stack Size are what you think they are. The Attack Modifier is a separate calculation based upon the hero's Might or Magic Attack value, depending upon the type of damage the creature inflicts. The Raw Damage is calculated as a range, due to the Creature Damage not being a fixed number.
The second calculation brings in the defense of the stack being attacked:
Final Damage = Raw Damage * (100% - Resistance %) * Misc Modifiers
The Misc Modifiers are bonuses given by spells, skills or artifacts (i.e. friendly creatures' ranged attacks deal 8% more damage). Resistance % is calculated in a similar manner to the Attack Modifier using the defending stack's and hero's Might Defense or Magic Defense.
Final Damage is also a range (as Raw Damage was) and the calculation then picks a random integer out of that range for the actual damage caused by the attack.
As to your comment question regarding +3 Might Defense, that does mean +3 to the the Might Defense value, which is turned into a percentage during the Resistance % calculation.
Best Answer
From my experience, your morale stat is expressed simply as a percent chance of a morale effect. Positive morale gives a second turn at "half strength" i.e. half movement and half strength attack. Negative morale forces the unit to miss its turn entirely. So if you have a +28 morale, you will have a 28% chance of gaining a half move/attack. There can be other situational effects of good morale, such as the Haven faction's ability bar filling up, or triggering additional turns of persistent effects like regeneration.