Though don't dismay as if you missed your states deadline (you HAVE to register to Vote From Abroad from the last state you were registered to vote and with that address [even if it was 20 years ago] and the new LAW in MOST states is you MUST RE-REGISTER with EACH ELECTION if you're Voting From Abroad) you can always do the Federal Write-In Absentee Ballot (FWAB) where you'd ONLY be voting for the President.
The FWAB is a back-up ballot that you can use if you requested your ballot 30 days before the election OR by your State's voter registrations deadline, WHICHEVER IS LATER. You should use the FWAB if you have not received your pre-printed state ballot 30 days before the election.
To answer more of your questions (since it's a bit complicated) AND TO REGISTER TO VOTE FROM ABROAD - go to:
PLEASE "SHARE" this with EVERY American you know that LIVES ABROAD - or knows someone that lives abroad