Go to a channel for serving songs (#mp3 #mp3_blackdeath #mp3-deathmetal).
Type @locator followed by what you are searching for. (Ex. @locator Opeth OR @locator Iron Maiden Remember Tomorrow)
If people have the song, they will automaticaly reply with a send command and the file name. (Ex. !ChannelUser Opeth_-_Still Life - 03 - Benighted.mp3)
Copy this file name (from the exclamtion point to the end of mp3) and paste it into the channel.
Depending on how busy that users server is, you will either recive the file immediatly or after you are up in que.
I know that wasnt the best description, but I'm not a very good explainer, hopefully this will give you a better understanding though. It's ALOT easier than it sounds.