All the registered users are allowed to add a new topic to the discussion forums to which they have access. Which means that though you might be able to access discussion forums for other courses, you might not be able to add new topics there.
To add a new discussion topic, go to the concerned discussion forum and click on “POST NEW ITEM”. In the new window that opens up, you can either type out your data or upload it from an existing document. You can navigate through the Discussion Forum using the “JUMP TO” option.
In order to set up your personal preferences for the Discussion Forum, please go to MY PREFERENCES and click on FORUM PREFERENCES. Once you have chosen the preferences, click on SAVE and the preferences will be applicable till you change them again.