After opening the report definition click the button. You will see the New Variant screen:

New Variant screen
You must enter the exact Name of the parameter, as defined in the report definition, and assign it a Type. Above we see the sold_param defined as a type "Bool" (boolean) which means it has a true/false value in the database.
After clicking the button, you will be prompted to enter a value for the parameter and check whether or not it is active.