Carole,
Some forums are News forums that are only available for staff to post on. These are intended for course announcements not for general discussion. If you think that it is intended that you should be able to reply on a particular forum then please raise it with your tutors on the course to determine if the wrong type of forum has been used.
As for your video issue can you supply details of the operating system and browser that you are using and a screen shot of the page showing the black screen.
Wesley