Aidan,
If you want to change an existing page, you can change this in the Properties pane on the right side of Studio Pro:
If you are creating a new page, select the Layout using this drop down box in the New Page popup:
Hope that helps,
Mike
Found the solution:
Unlike most other widgets, the name of a placeholder carries a special significance. A layout contains one or more placeholders and one of these placeholders must be named Main.
I added a placeholder but it wasn't called 'Main'. Once I changed the name to 'Main', the page appeared as a navigation option when creating a new page.
https://docs.mendix.com/refguide/placeholder/