You haven't done anything wrong. You're about half-way through the setup of the channel at that point. Please make sure you're referring to our
Joomla social networking posting guide
.
After that step, the user that you are configuring the channel for needs to authenticate on the front-end of your site again. They should receive an additional permissions box allowing your app/site to post to the group they manage on their behalf. After that, you can edit the channel and select which group you're actually going to be posting to.
I hope that helps explain, but if not, let us know.
Thanks,
Alex