The SciFinder Web structure editor allows users to draw structures for substance and reaction searches. There has been a new structure editor available since October 2013 that does not require a Java plug-in for web browsers. It is now set as the default structure editor and the most features are implemented.
See latest news at http://www.cas.org/products/scifinder/whats-new-in-scifinder
Alternatively you may use the standalone CAS structure editor.