1. Just a few sound cards don't support Stereo Mix.
If you can't find Stereo Mix (or 'Mixed output', 'Post-mix', 'Loop back', 'SUM' and the like), so you can't record computer sounds
2. Install the proper sound card driver.
If you installed a wrong sound card driver, you can't find Stereo Mix
3. Make sure that you clicked the Recording radio icon of Properties.
Some people click "Playback" radio icon to find the Stereo Mix