Models navigation is setup so that you drill down through three levels until you reach the models.  A convention of how to navigate items had to be reached as having thousands of them in one big pile would be an unreasonable situation.

The model hierarchy is setup as follows.  The top level is category.  The level below category is called group.  The level below group is called division.  Once you have chosen a division, you reach the models that can be placed in the tray.

Begin seeing what models are in a given hierarchy, begin by selecting a category below.