You're using a development snapshot, which contains changes leading up to Tiled 1.5. This includes the removal of the Terrain Brush, since this functionality is now completely covered by the Wang Brush and the related Wang Sets mode in the tileset editor. Unfortunately I haven't gotten around yet to updating the documentation, but the workflow remained largely the same.
If you prefer to go back to a version where you can follow the steps in the video exactly, you should use Tiled 1.4.3 instead.