You can view and select the scenario variables currently defined by the project using the Scenario View.
To enable the this view, use the Window menu entry and select "Show View" -> "Other..." to bring up the dialog box shown below. Expand the "Ada" category and select "Scenario Variables".
This will open a view depicting the external scenario variables defined by the project, allowing you to chose the values to apply to them. The name of the variable appears to the left of the pull-down list, with the possible values for each variable in the corresponding pull-down list.
