How can i change the car tittle to my name ? plz help?

how can i change the car tittle to my name, i just just bought an used car, i had the seller signed and filled all the information in the back of the tittle. i took it to the DMV and been told that my signature and the seller's signature must be notarized first. i dont know what to do now because i lost the contact with him. does anybody have any idea or suggestion ? it's be greatly appreciated