Try to find the address to her agent andor the company (probably Paramount) she's working for. Remeber to send stamps or a postal coupon for return mail. Good luck!
The best way would be to go to a convention where she will be signing. She will be at the Vegas con this year. I've seen her several times and she is very nice
Of course, the best way would be to meet her during a convention or on sets of a show for which she would direct an episode but, you can write to dana@roxanndawson.net, to ask how to get an autographed picture (http://www.roxanndawson.net/email.php) Her official website is http://www.roxanndawson.net/ Good Luck! :-D