Global fields can be defined in any table in your database and will still be accessible from any layout and any script in your file. The only time you need to put a global field in a specific table is if it is used as the match field in a relationship.
Thus, you may want to add a "globals" field to your database as the place to put all of your global fields.
If you are using Filemaker 11 or newer, you can pull down the Insert Menu while in layout mode and use the Merge Variable option to add a merge variable to your layout. This puts <<$$>> on your layout and you'll then need to type in the name of your variable.
If you are using an older version of Filemaker, you can't display a variable's value on a layout and you'll need to stick with global fields.
I had created a "globals" table, but neglected to define the fields as global fields. Once I did that, they displayed on the layout.