You can view and modify the scenario variables currently defined by any project using the Scenario Variables 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".
In the Ada perspective, you can also open the view using the Window menu entry:
This will open a view depicting the external scenario variables defined by all projects, allowing you to chose the values to apply to them. The external names of the variables appear underneath their defining project names, with the possible values for each variable in the corresponding pull-down list. You can click on the value to invoke the pull-down list, or click on the down-arrow at the right of the value.
If a given project does not define any scenario variables, none will appear underneath the project name. Projects that are not Ada projects will not appear in the view at all, nor will those Ada projects that are not root projects.