Recording what's playing on iMac

Hi,

I have just got Ultra Mixer for the mac and need to know how to record the music that is playing through the speakers onto my mac.

I am trying to mix various songs together and record them as an mp3/wav but can't seem to find out how. There are various programs that record from the mic or line in but I can't seem to get them to work for what I want.

Any idea's??

Thank you