Pixelmon Mod

I downloaded a new soundpack, but I don't know how to change it. Where can I find the file and how it's called. Can someone please create a short description with pictures or do a short video. I seached long for help, but I couldn't find anything. You're my last hope :D
Thank you for your help,
MakkAnger.

(and sorry for my bad English, I'm from Germany >.<)