AA is correct, in iTunes
Syncing the iPod is a one way street
However, you can use 3rd Party software to recover your music from your iPod, refer to this earlier post by Zevoneer,
or download CopyTrans to do it.
have a nice day!
(I realize this post is very old, but for anyone who comes looking for a solution, I hope this helps!)
I haven't done it in a while, but I believe it can be accomplished by the following (this is for a Windows computer...I don't know about on a mac):
1. Make sure you can see hidden files and folders on your computer. (look for "Folder Options" in your control panel)
2. Connect your iPod (but don't let it sync!)
3. Navigate to your iPod through My Computer.
4. Find and then copy all of the folders containing music to your computer. Then copy/move them to Music/iTunes/iTunes Media/Automatically Add to iTunes, or click File>Add Folder to Library in iTunes, and select the appropriate folder(s).