You cannot transfer music from iPod to computer without an application such as Senuti, which should work fine if you have a classic or nano (it might work with a Touch, although it didn't when I tried, but I'm guessing they've brought out a workaround by now). Or if you have a classic, then enable it as a hard drive (doesn't work with the Touch).
However by far the easiest way is to turn on file sharing in system preferences, and transfer the music across over the network, or do you have a external hard drive?