That's good you figured out most by your self.
Easiest trigger I can think for a chance to drop item would be this:
Events
Unit - A unit Dies
Conditions
(Random integer number between 1 and 10) Equal to 1
Actions
Item - Create {item} at (Position of (Dying unit))
If creeps need to drop random items and you know how to work with variables then you need 2 triggers.
trig1
Events
Map initialization
Conditions
Actions
Set Item[1] = Claws of Attack +15
Set Item[2] = Crown of Kings +5
Set Item[...] = .....
Set Item[10] = Orb of Frost
trig2
Events
Unit - A unit Dies
Conditions
(Random integer number between 1 and 10) Equal to 1
Actions
Item - Create Item[(Random integer number between 1 and 10)] at (Position of (Dying unit))