Executing the original code generated a window with only one widget (see http://i.imgur.com/JALe6q4.png). The problem was that I called the parent class within the super function.

TestPlan.py

from kivy.app import App 
from kivy.uix.gridlayout import GridLayout
from kivy.uix.button import Button
from kivy.uix.textinput import TextInput

class TestPlan(GridLayout):

    def __init__(self, **kwargs):
        super(TestPlan, self).__init__(**kwargs)
        self.cols=2
        self.add_widget(Button(text="Description"))
        self.username=TextInput(multiline=False)
        self.add_widget(self.username)
        self.add_widget(Button(text="New"))

class Test(App):

    def build(self):
        return TestPlan()

Test().run()