After you wipe out all of the Drengin on the third map, there should be a popup telling you to lower the shield on Earth.  At that point the "helper" arrows should be pointing at a tile very near Earth.  Send the Crusader to that exact tile, sit back, and watch the show.

I have the same problem ... The arrows are directly over Earth there is no waypoint

Actually there are several optional goals in the campaign of the last (third) map. After conquering Mars you can descide wheater you want to go on with the war against drengin and wipe them all out or you can just bring the big ship into a "rally-point". But first you have to conquer Mars (at least that was how i won the game...)