Real Time SQL Monitor

SQL Developer - Tools - Real Time SQL Monitor
Select Connection
Then select which database you want to connect to and monitor.

SQL Developer - Choose Connection
SQL List

SQL Developer - Real Time SQL Monitor
DBA Panel
In fact, the "Real Time SQL Monitor" is one of the function from "DBA" panel. The navigation path is as below:
DBA > Connections > Tunning > Real Time SQL Monitor

SQL Developer - DBA Panel - Tunning - Real Time SQL Monitor