Use Entry.insert. For example:
try:
from tkinter import * # Python 3.x
except Import Error:
from Tkinter import * # Python 2.x
root = Tk()
e = Entry(root)
e.insert(END, 'default text')
e.pack()
root.mainloop()
Or use textvariable option:
try:
from tkinter import * # Python 3.x
except Import Error:
from Tkinter import * # Python 2.x
root = Tk()
v = StringVar(root, value='default text')
e = Entry(root, textvariable=v)
e.pack()
root.mainloop()