If you go on the BBC website and click on LIVE, they have a list of all the events that are available to stream online. Alternatively, follow the link to the sport you are interested in and you can get to the stream through that page. However, the links aren't available until they are actually on air, so don't dispair if you can't see them 2 minutes before the start. I've watch all the Eventing, Dressage and Showjumping so far... been brilliant to be able to watch so many horses at top level when usually its so difficult to see them
On a separate note, if you are not in the UK the streams won't work. However I've got a way to get them to work, PM me if you are overseas and need the details of how to do this.