Just a little script to see kivy's ProgressBar working within a Popup window.

kivy_progressbar_example.py

import kivy

from kivy.app import App
from kivy.clock import Clock
from kivy.uix.button import Button
from kivy.uix.popup import Popup
from kivy.uix.progressbar import ProgressBar
from kivy.uix.widget import Widget
from kivy.properties import ObjectProperty


kivy.require('1.5.1')


class MyWidget(Widget):
    
    progress_bar = ObjectProperty()
    
    def __init__(self, **kwa):
        super(MyWidget, self).__init__(**kwa)
        
        self.progress_bar = ProgressBar()
        self.popup = Popup(
            title='Download',
            content=self.progress_bar
        )
        self.popup.bind(on_open=self.puopen)
        self.add_widget(Button(text='Download', on_release=self.pop))

    def pop(self, instance):
        self.progress_bar.value = 1
        self.popup.open()

    def next(self, dt):
        if self.progress_bar.value>=100:
            return False
        self.progress_bar.value += 1

    def puopen(self, instance):
        Clock.schedule_interval(self.next, 1/25)

class MyApp(App):
    def build(self):
        return MyWidget()


if __name__ in ("__main__"):
    MyApp().run()