How do you address a letter to an Ambassador designate?

Best I can do on this one: Since the person is not officially an ambassador yet, you would address the letter as follows:

Dr. John Smith

Ambassador Designate to Oklahoma

<<at his present address>>

In the greeting of the letter, refer to him by his present title: Dear Dr. Smith, etc.

In the opening of the letter, it is always proper to congradulate him on his appointment. After his becomes official, then you would address the letter to:

Honorable Ambassador John Smith

Embassy of the United States

123 Any Street

Omro, Oklahoma 12345

and in that greeting: Dear Honorable Ambassador Smith...