Combine 7-up and marshmallows in saucepan and cook over medium heat until marshmallows are melted. Remove from heat and add gelatin, stirring well until dissolved.
Add 7-up mixture to the softened cream cheese and beat until smooth. Stir in the pineapple and walnuts. Chill until partially set. Fold in whipping cream ( do not whip the cream) and mayonnaise or salad dressing. Pour into a 9-by-13 inch pan that has been sprayed with oil. Place in refrigerator until set.