Assuming that the map is In Line With Text, you can draw a circular
AutoShape on top of it (press Shift to constrain the Oval tool to a circle,
Ctrl to draw the circle out from the point you want to circle). The wrapping
on the AutoShape will probably be Square by default, which will make your
map do something wonky, but don't panic! Just double-click on the AutoShape
to open the Format AutoShape dialog and, on the Layout tab, change the
wrapping to In Front of Text. The AutoShape will also, by default, have a
white fill, so on the Colors and Lines tab, choose No Fill. You can also
adjust the line weight and color as desired.