"I did this one thing, and now my Mac shows a folder with a "NO" sign" thread in 3...2....1....
OP, I suggest you Google for a basic introduction to using a command line in UNIX, you need to learn the basics of getting around first. After that, study up a bit about permissions and the sudo command. Blindly going in with no clue what you are doing is a good way to break your whole system. Then we will laugh at you and tell you to do an archive and install.