First of all startup your database.
Then connect to your catalog database.
DOS> rman target sys/oracle catalog rman/cat@CATDB
where sys/oracle are username and password for the database you're registering,
rman/cat are username and password for the recovery catalog user and CATDB is the service name of recovery catalog database.
From the RMAN command line simply type
RMAN> register database;Your database is now registered to the recovery catalog database.