How to take input from Tkinter

What you are looking for is [widget].get()

Text widget

In case you use the Text widget, you have to use [widget].get(1.0, END) where 1.0 means "first line, 0th character"

Code review

I've noticed a few other things in your code that could get improved:

• PEP8 conformity; see pep8online.com
• If you add a Shebang, Linux users will be able to execute it directly with ./script.py.
• Variable naming:
  • input is a built-in function and you should avoid overwriting it
  • Use meaningful variable names (entrytext might be problematic in case you extend your program)
• Avoid from Tkinter import *. This might lead to unexpected naming clashes.

Complete code

##!/usr/bin/env python

import Tkinter as Tk


class App(object):
    def __init__(self):
        self.root = Tk.Tk()
        self.root.wm_title("Question 7")
        self.label = Tk.Label(self.root, text="Enter your weight in pounds.")
        self.label.pack()

        self.weight_in_kg = Tk.StringVar()
        Tk.Entry(self.root, textvariable=self.weight_in_kg).pack()

        self.buttontext = Tk.StringVar()
        self.buttontext.set("Calculate")
        Tk.Button(self.root,
                  textvariable=self.buttontext,
                  command=self.clicked1).pack()

        self.label = Tk.Label(self.root, text="")
        self.label.pack()

        self.root.mainloop()

    def clicked1(self):
        weight_in_kg = self.weight_in_kg.get()
        self.label.configure(text=weight_in_kg)

    def button_click(self, e):
        pass

App()

Is this the kinda thing you are looking for?

from Tkinter import *
import tkMessageBox

class App(object):
    def __init__(self):
        self.root = Tk()
        self.root.wm_title("Question 7")
        self.label = Label (self.root, text= "Enter your weight in pounds.")
        self.label.pack()


        self.entrytext = StringVar()
        Entry(self.root, textvariable=self.entrytext).pack()

        self.buttontext = StringVar()
        self.buttontext.set("Calculate")
        Button(self.root, textvariable=self.buttontext, command=self.clicked1).pack()

        self.label = Label (self.root, text="")
        self.label.pack()

        self.root.mainloop()


    def clicked1(self):
        input = self.entrytext.get()
        result = int(input)*2
        self.label.configure(text=result)

    def button_click(self, e):
        pass

App()

I think this is what your'e looking for, although not just times by 2. You would probably also want to put in an exception for if the value is not a int.