Selenium- how to automate download in http://ift.tt/1rbWxtg

I am using selenium to automation the downloading of from a website:

http://ift.tt/1rbWxtg

There is no problem for me to click 'OK' in the first page, but I cannot click 'Download' in the second page.

I tried find_element_by_xpath, find_element_by_class_name ... none of them work. Could anyone familiar with Selenium help me solving this problem?