1. Restore controlfile from autobackup:

RMAN> set dbid=nnnnnnnnn;
RMAN> restore controlfile from autobackup;
 
2. Restoring from autobackup when the backup location is not default. 

RMAN> set dbid=nnnnnnnnn;
RMAN> set controlfile autobackup format for device type disk to '/tmp/%F';
RMAN> restore controlfile from autobackup;
 
3.   Restore the controlfile from this specific autobackup. 

RMAN> set dbid=nnnnnnnnn;
RMAN> restore controlfile from '/tmp/c-3344771490-20120502-03';