To find a channel that is dedicated to MP3 exchange, do a channel list with Cmd-L and enter "mp3" in the Include field and a reasonable minimum number of members, like 20 or so. You can also use the Find and Find Again functions to search in an existing list. To show the request list, press Cmd-P.
Once you've entered a MP3 channel, you will usually see many messages scroll by very fast. These are typically requests sent by someone to another channel member that happens to have songs available for download. The traffic is typically very high, making it hard to find things and discover who has what songs available.
A typical request is in the form "!nick song". If nick has the song available, he will then start sending it to the person that sent the message.
The MP3 request list will filter out the song requests that people send to the channel so that you can also request those songs. In a couple of hours in a busy MP3 channel the list may pick up thousands of available songs that you can request with a click of a button.
To specifically search for an artist or song that interest you, most channels support a "@find" or "@locator" function. Type @locator and the phrase you are searching for and send it to the channel to ask who has the song available for download.
If someone has it, he will send a private message with instructions on how to retrieve a song. The MP3 request list will pick up this information and you can then easily download the song.
Clicking the Get button will send a normal "!nick song" message to the channel, and if you are lucky the offerer will start sending you the file. If this doesn't happen right away, the offerer may be busy with other clients and will send the file to you later.
Most of the people offering songs are running a particular script that responds to these messages automatically so they have to be formatted just right. The MP3 request list makes that easy.
Most channels place limits on how frequently you can request files. Try to not send a message more frequently than every few seconds. The list supports multiple selection, and will send multiple requests to the channel if you do that and press the Get button.
The songs are sent to you as DCC file transfers (see the DCC chapter for further information) and it may be useful for you to set up an Action to automatically accept the MP3 files and put them in your iTunes folder.
This is useful because the offerers may have you waiting in a queue for a while for a popular song that many people have requested. When it finally gets to be your turn, you may not notice that the offerer is now trying to send you the file and it will time out.
Because the request list may quickly fill up with a large amount of songs, you can enter words that you are especially interested in, in the highlight field in the window. When a song in the list contains one of the words it's highlighted in red color in the list so it's easier to see.
If you want to only retain songs in the list for a certain amount of time, you can specify a timeout value in minutes in the General preference panel. The song is then automatically removed from the list after that period of time.