How many line-outs could there be? Just try it again and again until it works. If it doesn't, go to the preferences both in Audition and your computer, make sure everything is set, and keep trying the different line-out plugs. If it still doesn't work, then you're probably bat-sh** out of luck.
We still need to know what the OP is using for audio out of his computer and what operating system it is using before any useful advice can be given. At the moment we can only assume that it is a Windows PC of some sort and that he might be using an on board soundcard.