Ok
So First You Must have a mob you plan to do this with....ill use
a Flesh Eater
Ok first go into your database of creature names on navicat and select the your mob mine is then find the ID mine is 3 (using chrispees repack) now remember ur Mobs ID
And Then go into items and find the item you wanna add to the creature anythingu want then find the items ID and remember that
Then Finally Go into your Creature loot table and Add a new one by clicking the plus button or w/e and when putting it in make the first part where u can put text in a very high number 600000 should work just so it doesnt interfere with the orignal one
then go to the next one and put your mobs ID in it Then go to the next one and put you items ID in it
Now this part is ur desicion put whatever precent u want this to drop so if u deffiantly want it to drop everytime put it at a 100 if u want it to drop little then make it 10 its ur choice put the next one at zero and the last to 1's then save it and restart ur server then test the mobs see if it works if it does then well u do it