Most M/Bs that truly have that capability, come with an additional connector adapter, that plugs into the M/B, and secures to the PCI connector plate, that adapter has 1/8" stereo jacks for Rear Output, and Center/Sub Woofer Output, you get your Front Channel from the fixed Line Out on the M/B itself. Nine times out of ten if your M/B did not come with that adapter, that particular M/B doesn't have the 5.1 capability, even though it may be mentioned in the M/Bs manual, because the manuals can cover more than one series of M/B model numbers, and usually has sub-notes that indicate which models have what capability. If you list your M/B manufacturer and exact model number, I'll find out for you whether you have that capability or not, or you can find it out through the manufacturers website yourself.
Details, Details, Its all in the Details, If you need help, Don't leave out the Details.