nUI should show a vehicle frame not attached to the map according to a few posts. I don't have the mammoth yet myself so I can't test it.
From the other post's it looks like there may be two possible things happening.
1. The vehicle frame may be at the top of the screen. Type "/nui movers" into chat and locate the frame first to make sure it's there where you can see it and know where it will appear.
2. You can right-click on the passengers to eject them once you can see the frame.
Hope that helps
Source:
http://www.wowinterface.com/forums/s...hlight=mammoth