You can reach the Blocking Locks window via the Enterprise Manager and the Activity node. Double clicking the Blocking Locks node will open a window similar to the one below.
Blocking Locks - Locks Per Process
The windows holds information about who is blocking who and with what kind of lock on which object.
The Locking User is the user holding the lock. The locking session can be killed (so that other sessions can continue) by using the Kill Locking Session button. Be careful though, any uncommitted changes for this session will be lost.
The window will automatically refresh itself each number of seconds as is shown in the spin-edit, unless the Paused check box is checked.
Note:
The user you used to register the server needs to have access to views V$SESSION, V$ACCESS and V$LOCK for this feature to work.