In my case I was trying to import email from my existing POP3 account before I switched the mail provider to Google Apps. Google Apps did not like the fact that I was trying to add the email account that I was logged into. If this makes sense you are in the right place and I have the answer for you. Make up a bogus email address to get past the screen, on right, that is giving you a problem.
Once you get past that screen enter in the proper credentials and Gmail will bring in the emails. You may want to leave the emails on the old server, I did, so make sure to check the “Leave a copy of retrieved message on the server”
Good Luck. -CB