First you have to create a "integer" variable, then you make a event with "Unit Dies" and in condition you choose the unit that drops the object, then in "actions" you choose the option "Set Variable"- Random Integer Number Between (% drop chance) and 100 or 1000 if you want to make it with decimals. and finally you go to If/ Then/ Else. option in "If" you put an integer comparision with "less than" you write the % drop chance of your unit. In "Then" you put "Object-Create in position of dying unit"
and in "Else" you simply put "Do Nothing"