Overview:

A custom button is created for a related list used by EM Event Page Layouts. Users are given appropriate visibility to the button.  It is also marked as a Selected Button in the Related List Properties for the page layout. However, the button is still not visible. 

Answer:

This is expected behavior. As of this time, the Event Management Page Layout Engine does not support the use of custom buttons for objects on the Related Lists . 

Related Documentation:

CRM Help Documentation: Defining Page Layouts