In UML 2.5.1, Visibility is a property of Named Elements. It is used to manage Namespaces. It is indicated by a single character on the left of the named element. A Generalization is not a Named Element so a Visibility is not relevant for generalizations in UML.
Private and Virtual inheritances are not Visibility properties and not UML concepts either. These kinds of inheritance may however be modeled as Constraints that are indicated in braces in UML.
With StarUML, you cannot add a constraint directly to a generalization which too bad. You have to add a constraint in your model with the Model Explorer and then move it under your generalization. You put the "-" or the "virtual" in the specification property and select your generalization as the constrainedElements property then drag the constraint in the diagram. It will automatically be attached to the generalization.
I hope it answers your question.