Setting Up SCD Type 2 using "Variable to set date values"

I am creating an SCD type 2 dimension and using start and end dates to identify current and expired records. In my destination table, there are start date and end date columns. I am unable to select the variable to set date values in the SSIS package. When I click in the "Variable to set date values" box, I don't see a drop-down to select my system variables from. I want to use the System::ContainerStartTime. Do you think this is something I need to configure in the SSIS package? If so, how?