Importing analog audio from reel-to-reel into Audition

Simple answer: connect the Line Output from your reel-to-reel machine to the Line Input of your computer, make sure Audition is able to connect to that input and start recording, preferably in multitrack view.

Rather fuller answer: you really need to be using some kind of good quality (external) audio interface, preferably with an ASIO driver, so that you can bypass all the "processing" that Windows might decide to try and do to your sound!  As above, you would connect the Line Out of your recorder to the Line In of the interface and then the interface to your computer, most commonly today, via USB.  

If you tell us rather more about the actual equipment you are using we may have additional advice to offer.  It might also be helpful if you were to describe your level of experience/expertise with Audition.