What is the proper salutation to be used in a letter to a consulate?

The highest office in a Consulate is a Consul, the highest office in a Consulate-General is a Consul General, and the highest office in an Embassy is an Ambassador.

You would address the letter as:

The Honorable John Smith, Consul

Consulate of "conventional long form of whatever country"

Street address

City, State, zip

In the letter however, the salutation would simply be Dear Mr., Mrs., Miss, or Ms., the same goes for a Consul General

A foreign Ambassador would be:

His (or Her) Excellency John Smith

Ambassador of "conventional long form of whatever country"

Street address

City, State, zip

The salutation for an ambassador would be either Excellency, or Dear Mr. (or Madam) Ambassador