It seems that SQL Developer thinks that you are running a plain DML (data manipulation) script, not a DDL (data definition). It also thinks that :new.id is a bindable variable.
Why this happens, I don’t know; I can’t reproduce it in Oracle SQL Developer 2.1.
Try to open a new SQL worksheet window in the theschema schema and execute a “whole” script (not a statement) by pressing F5 (not F9).