If you don't know how to address a cover letter, then you don't have enough information. A cover letter will ALWAYS be more powerful and effective when it is personally addressed to someone.
You SHOULD be able to get this information in most cases. A call to the receptionist at small to mid-sized operations will probably net you the name you need. Just ask something like, "I'm sending in my resume in answer to the ad in the paper, and I want to be sure it gets to the right person. Can you tell me the name of the recruiter?" (Or the Director of HR, etc.) If you're calling a larger organization, you may want to ask to speak with the HR department and then pose your question to them.
If you absolutely can't get the name and title of the person doing the hiring, then you can just address your letter to Dear Recruiter. But that is not likely to be as effective, so I'd do your research whenever possible. The effort you put into it will be well worth it when you get the call for that job interview!
