The fields in getAddInput need to be called info1, info2, info3, and info4 (there's a maximum of 4). Also, the function should be coded like this:
function getAddInput()
{
global $item_info;
if ($item_info[1] == 0)
$item_info[1] = 100;
return 'Amount to decrease by: <input type="text" name="info1" value="' . $item_info[1] . '" />';
}
If the user is editing the item, the current values are stored in the $item_info array. If they're adding a new item, the values in this array will be blank, and you'll probably need to set a default (as shown in the code snipplet above).
To actually use the values when the item is used, you use the $item_info array in the onUse() function.
Hope this helps you