Create a INTEGER variable, lets name it <itemdrops> and set it to 100.
Create Trigger as follows...
Event- A unit dies
Conditions- (dying unit) equal to <the creep that drops item>
Actions- Set <itemdrops> to random number from 1 to 10
If/then/else- -If <itemdrops> equal to 1 -then create (item) on position of (dying unit) -else do nothing
If/then/else- -If <itemdrops> greater than 1 -then do nothing -else do nothing
This trigger drops the specific item from the creep you specified in the condition. The item has a 1 out of 10 chance of dropping on ground whenever the specific creep is slained.