After a job interview, ask for the interviewer's business card; it contains his contact information. Ask for an estimate of how long until the employers get through their applicant list. That estimate is the length of time to wait before calling them to inquire about a follow up interview.
It is likely many people are interviewing for the same position, so going through all the applicants takes time. Respect any time estimate the interviewers give before sending a printed letter. While some people prefer emails or phone calls, others are impressed by a handwritten note; it provides a more personal touch. In the note, write a few lines thanking the interviewers for taking the time to meet with you and bring up something important mentioned during the interview, as it might help them remember you. It may also help to include suggestions of ways to increase productivity or efficiency in the office.
In the absence of any physical address details, sending an email is acceptable. This method is very common for following up after an interview. Be brief and mention the same things you would mention in a handwritten note. Make sure to give your name, phone number, email and address.