To setup a channel, there are a few steps:
1) Setup your LinkedIn Application
2) Add the API/Secret keys to your site
3) Authenticate on your site using LinkedIn
After you've authenticated using LinkedIn, then your account should show up in the Attributes area to select a user. You can also check the JFBConnect -> Usermap area to ensure your account exists in there.
I'm assuming you've already taken care of steps 1 and 2. It's step 3, which should just be a matter of authenticating and approving your application on LinkedIn to access your profile that should be left.
Finally, please note that you must be approved for the LinkedIn Marketing Program to use the channel feature with LinkedIn. The Marketing Program is free to apply to, but can take a few days to get approval.
Thanks,
Alex