If you want to edit ALL files just do the quick way:
Extract all files to your current location. After that select all, right click on them, use properties and remove the "readable only" hook. Then your able to edit all files.
To edit abilities go to:
dataGameMechanicsCreatureCreaturesfactioncreature.xdb
lets look into a creature: example pit spawn. To give an existing creature abilities add them between (they are just an example):
<Abilities>
<Item>ABILITY_DEMONIC</Item>
<Item>ABILITY_VORPAL_SWORD</Item>
<Item>ABILITY_AXE_OF_SLAUGHTER</Item>
<Item>ABILITY_LAY_HANDS</Item>
<Item>ABILITY_ENERGY_CHANNEL</Item>
<Item>ABILITY_MAGIC_PROOF_75</Item>
<Item>ABILITY_IMMUNITY_TO_FIRE</Item>
<Item>ABILITY_FIRE_SHIELD</Item>
</Abilities>
To give new spells add between <KnownSpells> and </KnownSpells> some code in propper form: