Did you find the boat on the left side? The blue t-rex goes in that one in the demo and the red dimetrodon goes in the boat on the right. There's supposed to be an indicator arrow that pops up if you have a dino that belongs on a boat, kinda like this:
If you follow an arrow, it should lead you to the appropriate boat.
The red-x is a similar deal. Sticking a dino on a boat it doesn't belong on will give a red-x and throw it back out:
Lemmy know if getting the blue t-rex on the boat lets you win the level. I'm thinking this is an issue with the level goal not being intuitive :/