AvFan has it right, go to Mac Preferences and select Sound/Output - when you have your MacBook plugged into the 990, you should see it listed - just highlight and go!
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Outlaw 976
Outlaw 7500
OPPO-103
Sony Bravia 52" XBR
DirectV
Apple TV
Infinity Speakers
Home Built Sub+800 WRMS Amp
Blue Jeans and PPC Cables
Phillips Home Theater Power Conditioner
Wi-Fi "ac" connected