When you exit the Muppet Show theatre, take a sharp right. If you don't turn enough then you'll be facing some of the Phineas and Ferb stuff. You have to go around to the right of the P&F merchandise and you'll see Lightening and Mater.
If you won't be going into Muppets then just head towards Mama Melrose, keep going past Phineas and Ferb past their merchandise and you'll find them.
As others have said, they don't talk or interact. They shake a little and make engine noises. There's a photopass photographer there to take pictures. That's it.