I wouldn't. A single C+ in your freshman year of college (in math, especially) shouldn't be a big deal. An addendum will probably just draw attention to it.
I wouldn't do it for two reasons.
1. It's a lone grade freshmen year
2. I don't think learning you don't enjoy math is worth writing about. It draws attention to the fact that you've only taken that single math class, and that's how you did.
Don't explain it. That would just draw attention to it. If you are asked in an interview, the class schedule reason or even just studying for the LSAT works.