To enter a foreign mailing address, go to the Mailing Address & Phone Number section of Personal Info. and click Edit.
On the page that opens, "Where do you receive your mail?" click on the the drop-down box that says "U.S. Address". Then one of the choices you will see is "Foreign Address."
If you want to enter your state of residence as a foreign country, then in Personal Info click Edit next to your name. Then scroll to the bottom of the State of Residence drop-down list and you'll see "Foreign or U.S. Possession".
Your mailing address is the address you want the IRS to use in case they have to mail you a letter or a check. It does not have to match your State of Residence.