You can contact Jensen through his fanclub,

Jensen Ackles
c/o Jensen Ackles Fan Club
PO Box 850812
Richardson TX 75085-0812
USA

Or through this address,

Jensen Ackles
1122 S. Robertson Blvd.
Los Angeles, CA 90035

Or by sending something to The WB.

Jensen Ackles
c/o Supernatural
Warner Bros
4000 Warner Blvd.
Burbank, CA 91522

Or by sending an email to JensenAckles@studiofanmail.com

A few words of caution.
Include a photo if you're asking for a signed one. Also include a self-addressed envelope or for those outside the US an international reply coupon. Please remember that Jensen is a very busy man, he receives a lot of fan mail and does not have time to reply to each letter personally.