This is not an original stage direction, but it is justified by the "Above" occuring after the next speech-heading. The "Above" is an original stage direction; it appears in the Folio of 1623, which is the only source for Shakespeare's text.
If Pindarus is "above," he is almost certainly in one of the two galleries above the stage. Imaginatively, Pindarus is "higher on that hill," just where Cassius told him to go.
