I am playing the campain in the Big MT map, you know ....how in the Hello can I return back to the wastelands? Please give me a clue 馃槉

You can't until you finish the main quest and get the teleportation device. Once you get that you can equip and use it as a weapon to return to the theater

Before you started OWB, you were warned in a message that you won't be able to return to Mojave until you finish the main questline.