Some companies will not accept material that is not represented by a music attorney or a management company. Find out if you need representation before you send in your demo. If you can get representation your odds of success will increase. The music attorney or management company will want a fee or a percentage in exchange for their services. Ask about their track record and check into who they represent and what results they have delivered.
Addresses are available in books that are published each year.
The Internet is also a great resource for information about companies. Call the company if you do not know what or how they want to receive their demos. Send them what they are looking for and in the format that they request. Be polite and persistent. The demo needs to be as high a quality as you can deliver.