If you save the form into your company directory on the server, with the correct name for the paper style you are using, the template will be picked up automatically.
Go to File ... Printer Font Settings - select the customer tab, then change to invoices under the select a document option. Below that, is the paper style assistant. What paper style do you have shown? eg Plain, Single Sheet ? If something else is shown, and you only print on plain paper, change the settings to single sheet plain paper and your templates will now work. You may have to change this setting for each document type (if you want quotes, sales orders, credit notes, etc) if you want them all to print using the same template. You will also have to change this for each user.