-Edit- Nevermind, I'll get you a better tut, Hold on...
-First step, Go into ability editor, copy Item Damage Bonus, under Data - Attack Bonus change it to however much attack damage you want.
-Events-
A Unit aquires item
-Conditions-
((Triggering unit) Has item of type (Item name here)) equal to true
-Actions-
Unit - Add (Whatever you named Item Damage Bonus) to (Triggering Unit)
Its not much but hope it helped,