selenium click in iframe fails

[3Pinter]

hi Guys,

What I want: clicking on the 'sign in' button on this website https://sites.google.com/site/ActiveRumblers/deck
I can get the input populated with data, but the click action ... nope. What am I doing wrong.

from selenium import webdriver
from selenium.webdriver.common.keys import Keys
driver = webdriver.Chrome("C:/Python27/include/chromedriver.exe")
import time

URL = "https://sites.google.com/site/ActiveRumblers/deck"
driver.get(URL)

#ensure page is loaded
time.sleep(3)

#login form is 4 iframes deep
driver.switch_to_frame(0)
driver.switch_to_frame(0)
driver.switch_to_frame(0)
driver.switch_to_frame(0)

#got xpath using google chrome, inspect, copy xpath
driver.find_element_by_xpath('/html/body/form/table/tbody/tr/td[1]/table/tbody/tr[2]/td[1]/div/input[3]').click()
driver.switch_to_default_content() 

time.sleep(2)
print "done"