Why isn't there a edit button?

I haven't yet implemented this functionality. For now it is simply easier for you to delete a hull and add it again if see anything incorrect.

Why can't I add a hull number or name?

Chances are the hull number or number has already been added. Simply find the hull number or name you wish to add at the database page and delete it. Now you should beable to summit your gensheet with that hull number or name.