First go to admin -> plugins -> repositories, and enable soem types of repository.
Then try making a forum post.
So many things have changed between 1.9 and 2.0 that it's difficult to keep up. Anyway I have followed your advice and enabled the repository types. But I have found an interesting "bug" (or feature?).
Images that are inserted into forum discussions are only visible to the person who inserted them, which rather defeats the purpose. Should I report this in the tracker?
I've also found this when I use a teacher account to insert an image into a course topic using the flickr repository - logging in with another account (in this case a student) and the image is not displayed