if you are going to spawn a ship, 1st you have to right click on the map where you want the ship to start. Very important, ships wont spawn if they are closer than 20nm. This can make a slight trip in the game. go thru the properties and select the ship or ships you want in the random group. Now at the towards the bottom of the properties you will see a box call spawn or spawning interval. defaul is set at 24 hours, means you will have to wait 24 hours for it to spawn. set it down to 1 hour. Now make the group entrance date and time. If you mission starts at 8:00 and you want the ship to spawn at 8:10 then the group entry time has to be set at 7:10. hope this helps. also you will see group instances, i tink its set at 1000, so it will spawn a 1000 times then stop. I had a problem setting it at 1 so I did it at 10 and it was fine. I think because it counts the hours till start time so if started at 8, it has spawned 8 times already? also you can set the amount of ships that will spawn. then set your trigger for time and make an event objective instance select the random group and ad the trigger to that event. Also you dont have to go look for the ship when it generates. go to your map F3, and bring up your clock. when you see the time has come for them to spawn, you should see a blue or black line towards the group that has spawned. I have a map I've made, its called behind you, you can get it here
http://hosted.filefront.com/ddmobiltv/