I think if you press alt, and you have the mouse pointer now, select your map from you quick links which by default are in the bottom right corner of your screen, once you have the map displayed you'l have an orange arrow on the map indicating that you have a mission at its location, right click on the arrow and select activate from the drop down menu, this will then bring up the orange arrow you used before .
There is a possibilty though, someone else may have done the mission and thats why the arrow isnt there now ?
Let me know how it goes and if that dosnt fix it I'l try and work something else out
Destory missions will always have a waypoint, but exploration missions dont. You have to wander around the wilderness hunting out the animals you are required to kill.
If you press CTRL D to open the datapad (and urrrm click on the new data tab?) it will show you an icon for each mission you have. You can examine this to review mission details or activate / deactivate mission waypoints
you can also destroy the mission if you get your ass whooped and dont want to tackle them again