Where is the address located on a letterhead?

Depends on whether you mean the printed letterhead address. That would be at the top, in the center of stationery.

If you mean the address to whom you are sending the letter, it would be on the left side, several lines separating that from the letterhead address on top.

The inside address, you type the several lines for the name and address, then skip one line and put the date there, skip another line and put the salutation, "Dear Horatio," or Charlie, or whoever.