Do you mean that the waypoint appears as an orange rectangle in your HUD?You see that thing when you are in a tank/helicopter/plane (I quess a thing with a radar)
Other times (when you are a lone soldier or in a jeep) it's defined by difficulty settings (extended HUD info). It can't be turned on when in veteran mode.
Or do you mean the path showing on the map?It's toggled in the editor waypoint dialog. In the lower half of the dialog there are three options
"Never show", "Show in cadet mode", "Always show"...
Or then I got it totally wrong, but since noone had answered...