When creating users in Oracle , you then have to create a connection in SQL Developer, repeating the information.
So you would run something like this in the SQL Worksheet
CREATE USER dermo IDENTIFIED BY dermo;
GRANT CONNECT,RESOURCE TO dermo;
Then you would have to create a SQL Developer connection.
In SQL Developer 3.0 you can use the SQL Worksheet to create the connection for you.
CREATE CONNECTION connectionname(CREATE USER username IDENTIFIED BY password);
GRANT CONNECT,RESOURCE to username;