You might be right that it isn't implemented yet, but since it said so in the manual I assumed it was in. Otherwise it's pretty basic, a def of 10 means you roll between 1-10 in def. So if you have 2 attack and are attacking an enemy with 10 def, you have a 5% chance to score 1 damage.
(chance of him rolling a 1 equals 10%, chance of you rolling a 2 equals 50%, and that's the only rolls you will score damage on)
Now if you had 20 attack, it's a whole different matter. If you roll between 11-20, you're guaranteed to score damage even if he rolls the maximum ten. And with optimal rolls here, you will score a whopping 19 damage (0,5% chance of that happening...)
(chance of you rolling a 20 equals 5%, chance of him rolling a 1 is 10%)
And as Sagittary pointed out, dex doesn't give direct defence, it uses %. Basically every point equals 10% of the armors def. So 5 point in dex would give you 5 def with a 10 armor item, while 15 dex would give you 15. With a 20 armor item the def values would be 10 for the 5 dex, and 30 for the 15 dex character.
Same with weapons and strength.
Tomas