You simply click the star right next to the sender's email address in the message header. This files the address to your Personal Addressbook and the star turns yellow.

You can click the star again to edit the contact.

Thank you , but I've tried that and nothing has appeared in my address book. I just figured the star was a way to say "this is not junk mail".

The star has got nothing to do with junk mail.

Please describe exactly what you did.

The only reason I assumed the star was saying "this email is ok and is not junk" is because nothing appeared in my address book when I clicked on the it. I just downloaded Thunderbird onto my new laptop today and since I'm still trying to familiarize myself with the program, I assumed I had done something wrong...but apparently, I had not. I would love for this to work so I can avoid having to manually type in all of my addresses.

In case the question wasn't clear enough, where exactly did you click?

I clicked on the star on the left at the beginning of the subject line. It turned yellow, but the email address did not go to my address book.

Please re-read my first answer above.
Hint: you'll need to open the message.

I was not looking at the header. It's tiny, but I found it! Thanks.