Paul, configure the Oracle TNS entry (put all "something" into $ORACLE_HOME/network/admin/tnsnames.ora file so that thru sqlplus, you should be able to connect to the RMAN catalog DB) such way that sqlplus rman/rman@catdb should work. Once connection is successful, then try,

rman TARGET SYS/oracle@trgt CATALOG rman/cat@catdb

Good luck to you and your company specially !