With external repositories, you need to take four steps

1. Create the repository (Manage Software Channels -> Manage Repositories -> create new repository)
2. Create the channel (Manage Software Channels -> create new channel)
3. Connect the channel to the repository (Manage Software Channels -> select channel -> 'Repositories' -> checkmark the repository)
4. Import the GPG key of the repo (you can add it in the GUI if you have the key. For open build service repos, adding it via the command line is simple:
login as root to the SUSE Manager server and run /usr/bin/spacewalk-repo-sync --channel <channel> --type yum
(replace <channel> with the name of the respective channel) and spacewalk-repo-sync will ask you to import the GPG key.