When you extract the .rar file the contents of the other r00/etc files are also extracted. You're using a slim burner, and slim burners are rubbish in general, so you need to use good quality media in order to have a chance of a good burn.
You can find Verbatim and Taiyo Yuden blanks on many online stores, and Verbatim can also be found in the big retailers I think. As for +R or -R you'd need to ask in a forum dedicated to Wii where people have lots of experience with it.