Still can't see Stereo Mix
It may well be that stereo mix as a recording source has been disabled by your laptop manufacturer. The RIAA have been banging on about being able to prevent people from doing this for years now, and in many cases they've been successful, despite there being any number of legitimate reasons for wanting to do this. You could try setting Stereo Mix as being the default input, but if it still doesn't appear as a source, then you'll have been externally nobbled - nothing to do with Audition, and it's not that rare any more either.
There is technically a way around it using something called Virtual Audio Cable, but this is hard to set up. There are various other suggestions here as well: How to Record the Sound Coming From Your PC (Even Without Stereo Mix)