See Page numbering /header / footer problems Word 2007 - 2013 and follow the directions to restore the built-in ones.

To make your own building blocks easier to reach, you can put them in separate categories, starting the names of the categories with either a space or an underscore. That will put them at the top of the list.
You will likely want to give your own building blocks meaningful and distinctive names. If one is suggested by Word it is likely to be neither.
Where can Building Blocks be stored?