If you want to use ModMii, you need to use windows. It's possible to get it to work on other OSs by using apps like vmware or wine.
If you decide not to use ModMii, there are other softmod guides available, I recommend the complete softmod guide (https://sites.google.com/site/completesg/) because it's the only guide (other than ModMii) that doesn't host copywritten files. If you don't care about where the files are coming from, then you can check out the other stickied guide in this forum.