I have finished most of Dino Domain, but I haven't found the key to the locked door in Dino Domain.
What track is the key hidden on? Where on the track do I need to look?
The Dino Domain key is hidden on the Ancient Lake track.
It's located at the very beginning of the track off to the right hand side. It will be floating on an inclined ramp. As mentioned in the comments below, you'll need a blue balloon to get the boost needed to grab the key.
This video will show you where: