Just for information, this kind of slang is not used in the UK. Our nearest equivalent would be "I'm (not) up for that."

It means you don't want to do something or you don't agree with something. It can have a negative feeling or a disapproving feeling. I want to go walk in the part tonight. I'm not down with that. (I don't want to do that.) You can say the opposite, "I'm down with that", if you want to do it. Let's go to the movies. I'm down with that. (Yeah, I want to do that.)

Also, this kind of slang should also not be used in the USA either. It is so much better to employ a literalism, such as; "I do not agree with that." or "I do not enjoy that." or something similar.