I got the edition with all the dlc not too long ago, and when I played, after the first few missions, icons for the 3 dlc campains appeared, and I could do those at any time. I think it was after I got to the hideout in the southern part of the map. I didn't care too much for the mech dlc, but the sea base one gets you a really cool boat at the start, and at the end, you get a lightning gun with infinite ammo (it recharges itself). The jetpack wingsuit and the lightning gun make the normal game fairly easy. You can take out bases by flying up, popping your parachute, then frying everything from the air.