In the ArcMap Table of Contents, click on the feature class line properties (circled in red below).
The Symbol Selector dialog will appear. Double click in the Preview pane of the dialog (located in the upper-right corner).
The Symbol Property Editor dialog will now appear. Under the Properties pane click the down arrow next to Type and change the Type to "Cartographic Line Symbol".
After changing the type to "Cartographic Line Symbol", click on the Line Properties Tab (circled in red below) and select the two line option under Line Decorations (circled in red below). Click "OK".
Your lines will now be displayed in the map with flow arrows.
