Actually OP, you
could actually use an HDMI cable to connect to your receiver, provided it has an HDMI-in on it. An HDMI cable transfers both audio and video over one digital connection, so this way you would use your receiver's DAC to convert digital data into actual sound.
The reason why this might not be the best way to do it, is because you would send a video signal to your receiver as well. If you JUST wanted to use it for audio then – well – it would be a little redundant obviously. It could also have a negative impact on your Mac's performance as you're basically making it double its graphics intensity. It wouldn't be much of a problem in general day to day situations though – only ever with high resolution video or games.
As
ChrisA mentioned, it's probably easier (though potentially more expensive) to use an
S/PDIF cable. This is a very specific cable that runs from your Mac's audio ouput jack to your receiver's optical input. This method also uses your receiver's DAC. The cable can be a little bit hard to find if you're unfamiliar with audio cables. Either ask the guys at your local (Apple) store or take a look here:
Belkin 6' Digital Toslink Optical Audio Cable with Line-Out Adapter
EDIT: Just as another tidbit of info: you would only use your Mac's
internal DAC if you connect something to it with an
analog audio cable. A pair of
headphones for example. Your headphones don't have a DAC, so it uses the one built in to your Mac.