Writing an addendum really only works when you're trying to explain the difference between two scores, or if you REALLY have a crazy excuse for why you scored low (I think I read some post on here or TLS about someone taking the test in Korea and them closing an airport due to political unrest or something. That's legit). If you were to take the test again and do significantly better, then you might have a reason to write an addendum, but doing bad on your ACT isn't a good reason.
When you take your law school exams, you won't be able to use the "I'm a bad test taker" excuse to get a better grade on a 4-hour written exam, nor will it work when you have to sit for the bar.
If you really want to apply this cycle, send a few apps out, but I'd consider retaking the test and waiting a year.