how do i create a grade layer above all the clips on the Editor timeline ?
Above the editor timeline, click the "New Clip" button with the green plus icon. Select Grade from the popup menu. This will create a grade clip on a new track above all others on the editor timeline. Adjust the position and duration of the clip to your needs. In your case it seems you want it on the whole timeline. Just resize the clip to cover the whole timeline.
@NormanPCN beat me too it, but here is a couple of screen shots to help.
and to add a new Video layer, just right click on Video on and add new layer.