Thanks for your reply. Yeas I have installed Meta Box plugin and activated it. I have also tried installing the extensions individually. However, there are still no Meta Box items (including the Builder) appearing on the Admin Menu.
I can (via the “New” option on the top admin bar) create a new Custom Field etc. However, because Meta Box (builder) does not appear in the main Admin Menu (on the left side) I cannot edit any fields or custom posts that I create.