You're suppose to find the number of the load order of the mod the item is from right? For example, the load order for the mod that I'm trying to find is 12, and the form ID for the weapon from that mod is 03004680. I typed in additem 01204680 1, and it said the item could not be found. I've tried it with other mods as well, and I get the same results.
You should then have typed "12004680" for formId, though note that you have to type the load order in hexadecimal notation, where "12" is actually decimal 18 (1*16 + 2). If you meant decimal 12, the hexadecimal number will then be "0C", for a formID of "0C004680".
You can see the correct hexadecimal number to use in Wrye Bash or OBMM.