You simply install it. Stereo mix has to be supported in the drivers AND by the audio chipset. The only thing obvious that I can see is that you are using Win7 AND Realtek, which (at least for me) have been nothing but trouble. On top of that, you are using 64 which always seem to have issues, even for items that do work in 32.
Bottom line...DRIVERS. Install the latest drivers for your chipset. If Stereo Mix isn't already in the audio driver package, I wouldn't expect much.
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