Which external soundcard for 5.1 sound?

I have an iMac 27 "with an i5 processor. I like to edit videos using FCP X. Now I want to edit 5.1 sound. In order to monitor sound I have an external sound card, the optical output of the iMac is not sufficient and I mean to edit 5.1 sound in FCP X! \
But what sound card do I need?? In a training video from Larry Jordan I saw that you really need an external sound card. He advises in this video the M Audio Fast Track Ultra. This USB Audia Card is 'End Of Life' (the Netherlands). A company I wanted to order this card they offered me for the samke price the M Audio C600 (actually the successor to the Fast Track Ultra. A nice offer.
Yesterday I installed the card. As far as I can see I manages all items I get sound out of the card only it's stereo...I still can't edit 5.1 sound with FCP X. ....

Who knows what card do I need? Or am I still missing something in the C600? Also in MIDI / Audio configuration I set a 5.1 setup (speaker setup).

There is so little information about this topic (hardware for FCP X). Who has the key for me? Who has an audio card (USB or Firewire) with working 5.1 sound on FCP X (and can edit this!!!)
I'm actually now more than a year to getr this workling and I also get something frustrated....