The point of having a script like this is so that you can import it, and call its functions.
At the bottom of the script, there is an example of how to call it.
root = Tix.Tk( )
font = askChooseFont( root )
if font:
print font
You want to replicate this behaviour in your original program. To do this, save this file as a script called fontchooser.py. And then add the following lines to your original program:
import fontchooser
import Tix
...
root = Tix.Tk( )
font = fontchooser.askChooseFont( root )
Now that you have the user's selection stored in the font variable, you can change the font via the appropriate functions in Tkinter.
If you already have a Tix.Tk instance running, you can pass that through as the "root" variable instead.