Heat oil in large skillet on medium heat. Add chicken; cook and stir 3 to 5 minutes or until no longer pink. Stir in Seasoning Mix and water.
Bring to boil. Reduce heat to low; simmer 5 to 7 minutes or until most of liquid is absorbed, stirring occasionally.
For each tostada shell, spread with 1/4 cup refried beans and 1/3 cup chicken mixture. Serve with desired toppings.