IMac digital out help


I recently purchased an imac to feed my audio aero capitole and use it as the DAC. In order to get the digital out i was planning on going toslink and got a mini toslink adapter thinking that the headphone jack is hybrid. To my surprise the mini toslink is quite smaller that the headphone jack. Is my output not hybrid? Do i have the wrong adapter? Should I pursue a usb or firewire digital out instead? Any help would be great.
quaded
This is the adapter i used. Still confused...

http://cgi.ebay.com/New-Monster-2-5-Mini-to-Optical-Optic-Toslink-Adapter_W0QQitemZ220644555313QQcategoryZ32838QQcmdZViewItemQQ_trksidZp4340.m263QQ_trkparmsZalgo%3DSIC%26its%3DI%252BC%26itu%3DUCI%252BIA%252BUA%252BFICS%252BUFI%252BDDSIC%26otn%3D13%26pmod%3D370418866102%26ps%3D63%26clkid%3D6241498573553112923
the imac is brand new. The plug is way too small. Apparently it is 2.5mm which is smaller that the standard 1/8" headphone plug. I don't know what this plug will fit but not my MAC. Weird.
i did buy an adapter. Check out the link a few posts above. I just happens that the mini is smaller than a 1/8" headphone jack. It looks right, but I guess it's for something else. Bought a cable from Dedicated Audio already to get this overwith.