Pioneer AV Receiver connection for surround sound

How would I be able to setup for a 5.1 sound output from the iMac to a pioneer AV receiver? I am only getting a 2.1 output always. I used to play the same video with a 5.1 output from a PC. However, am not able to do the same with my new iMac (late 2010 model). I connected my iMac from the miniDisplay port to the receiver using a HDMI adapter.