Each one of those 3.5mm plugs carry 2 channels.
Method 1 (using the composition rca out's):
Get (2) mono rca to dual rca cords.
This will give your two Lefts and two rights... front and back (4 channels)
You'll also have to set up one of those 3.5 as center/sub to send to the sub.
Method 2a (use the 6 channel sound 3.5mm):
You have to use the software with the computer to determine what each of these 3.5mm hook-ups do (line in/mic, center/sub, right/left, rear right/rear left)
If you need a line in then that leaves you with two 3.5mm's left to program. You would probably do a 3.1 setup which means the hookup is programmed to be right and left, the other would be center and sub.
You can take the front channel signal and use a 3.5mm to rca cord. That will give you a right and left channel. You can then take a mono rca to dual rca cord to turn your right rca to two rca's and your left rca to two lefts rca's. Now you got 4 channel that you can send to your amp.
Now take the center/sub 3.5mm and send that to the sub.
Method 2b (if you don't need a line in)
Use the software to setup a 4.1 system. One 3.5mm will be center/sub, another front left/front left, and the last rear right/ rear left.
Use 3.5mm to rca cord and send the appropriate channels to the amp