Hi there,
This bug was reported a couple of weeks ago by someone else. I have managed to make the toolbar with the "New Entity", "New N:N Relation" icons on it disappear somehow.
The way I got it back was to close all windows panes in DM, close DM and re-open DM. Selecting Windows->Reset Windows To Factory Settings might also work. I will continue to play around to see if I can reproduce it at will.
Regards.
John.