Hi James,
To set up a manufacturer filter for all product display you have to follow these steps:
Go to Display -> Filters, then New.
In this page, you have to set the name of your filter.
Select the type, for example "Single Dropdown".
Choose the category, for all products, you have to click on "Root" and select "product category".
Check Yes for "Including sub categories", this step will activate the filter for all product displays.
On the right, in "Apply on" select "Manufacturers", and then you can define all the other parameters.