Experimenting with your example code for adding an element, I get "Fatal error: Call to undefined method Torro_Elements_Manager::register()" called by torro()->elements()->register( 'My_Element' );
It doesn't seem that the Torro_Elements_Manager class has or inherits a register() method.
I noticed the base elements use torro()->element_types()->register(), so I replaced elements with element_types and it seemed to work.
Maybe something's changed since you did the documentation, or am I missing something?