Unable to add new pages or sub pages

My notebook has a home and a page with 2 sub pages. I dragged and dropped text into sub pages. I have put in links by dragging and dropping the address from firefox.

I want to add some extra pages and subpages. I can not do it because the add new page and new sub page options are greyed out. I have looked at the properties of the notebook and they are set to read and write, but I think zim thinks it is read only. I am using it in Ubuntu 12.04. What should I do next so that new pages can be added to the tree?