Dear Cherry:

There are many difference offices in the Vatican. Without knowing the issue, I would not know how to refer you. A letter directly to the Pope is rarely the appropriate place to write. Any letter to the Pope's office would only be referred to the appropriate congregation or office and delay any response, if a response if given, by as much as six-nine months or more. 

Unless the issue is a complaint against your the Bishop, or is a matter where the Holy See has direct jurisdiction, your local bishop is the place to start. Even with a complaint about the Bishop, the place to write is not the Pope, but the Congregation of Bishops.

If you can be more specific on the issue that concerns you, perhaps I can direct you to the proper office.

But, if you wish to try to send a letter directly to the Pope's office, here is the address:

His Holiness, Pope Benedict XVI
Apostolic Palace
00120 Vatican City

A standard Salutation in your letter is:

"Most Holy Father" or "Your Holiness"

For the Complimentary Close you can go with something formal and traditional, such as

"I have the honor to profess myself with the most profound respect, your Holiness' most obedient and humble servant"

...or a more simple form:

"Your most obedient and humble servant"

God Bless,
Bro. Ignatius Mary